Story highlights U.S. Sen. John McCain visits anti-government protesters in Kiev

Calls mass protest "incredible display of patriotism" in tweet

The Kiev mayor is among four under investigation for excessive use of violence

Police estimate 57,000 people attend pro-government rally

U.S. Sen. John McCain took in the anti-government protest scene in Kiev's Independence Square, known as the Maidan, on Saturday, snapping pictures with his cell phone as he met with opposition leaders.

He called it an "incredible display of patriotism" in a tweet that includes an aerial photo of the square. McCain also said he was with the younger brother of heavyweight boxing champion Vitali Klitschko, who is one of the opposition leaders.

The Republican senator has been critical of Ukrainian authorities' use of force against protesters.

"If Ukraine's government thinks that brute force and the politics of fear can see it through the current crisis, it is woefully mistaken," McCain said in a statement last week.

Photos: Photos: Ukraine protests Photos: Photos: Ukraine protests Ukraine protests – Newlyweds Mikhail and Margarita Nakonechniy kiss in front of barricades on Independence Square in a gesture of support for pro-Europe activists in Kiev, Ukraine, on Saturday, December 21. Protesters have poured into the streets of the Ukrainian capital, angered by their government's move away from the European Union in favor of closer ties with Russia. Hide Caption 1 of 77 Photos: Photos: Ukraine protests Ukraine protests – Protesters fill Independence Square in Kiev, Ukraine, on Tuesday, December 17. Hide Caption 2 of 77 Photos: Photos: Ukraine protests Ukraine protests – A girl wearing a traditional Ukrainian wreath attends an opposition rally in Independence Square on December 17. Hide Caption 3 of 77 Photos: Photos: Ukraine protests Ukraine protests – A young girl stands next to police officers guarding the presidential offices in central Kiev on December 17. Hide Caption 4 of 77 Photos: Photos: Ukraine protests Ukraine protests – Protesters sit behind wood bricks in Independence Square on December 17. Written on the bricks are names of Ukrainian settlements whose inhabitants are taking part in the demonstrations. Hide Caption 5 of 77 Photos: Photos: Ukraine protests Ukraine protests – Flowers are stuck on barriers in front of Ukrainian Interior forces guarding Kiev's administrative district on December 17. Hide Caption 6 of 77 Photos: Photos: Ukraine protests Ukraine protests – Pro-EU activists march in Kiev on December 17. Hide Caption 7 of 77 Photos: Photos: Ukraine protests Ukraine protests – Pro-EU activists stand guard at barricades guarding a heavily fortified tent camp in Independence Square on December 17. Hide Caption 8 of 77 Photos: Photos: Ukraine protests Ukraine protests – Activists warm themselves beside a bonfire as they guard barricades at Independence Square on Monday, December 16. Hide Caption 9 of 77 Photos: Photos: Ukraine protests Ukraine protests – Pro-EU activists cook during a rally in Independence Square on Sunday, December 15. Hide Caption 10 of 77 Photos: Photos: Ukraine protests Ukraine protests – U.S. Sens. John McCain, center, and Chris Murphy, right, join Ukrainian opposition leader Vitali Klitschko during a mass rally at Independence Square on December 15. McCain told protesters seeking closer ties with Europe, "The free world is with you, America is with you." Hide Caption 11 of 77 Photos: Photos: Ukraine protests Ukraine protests – An estimated 200,000 pro-EU activists gather during a rally in Independence Square on December 15. Hide Caption 12 of 77 Photos: Photos: Ukraine protests Ukraine protests – People sing, shout slogans and wave Ukrainian and EU flags during a mass rally in Independence Square on December 15. Hide Caption 13 of 77 Photos: Photos: Ukraine protests Ukraine protests – Pro-EU activists wave flags around the monument to the founders of Kiev during a rally in Independence Square on December 15. Hide Caption 14 of 77 Photos: Photos: Ukraine protests Ukraine protests – A couple of pro-EU activists share a tender moment at a tent camp in Kiev on December 15. Hide Caption 15 of 77 Photos: Photos: Ukraine protests Ukraine protests – Pro-EU activists, one waving the EU flag, gather during a rally in Kiev on December 15. Hide Caption 16 of 77 Photos: Photos: Ukraine protests Ukraine protests – Smoke rises as Ukrainian protesters continue their anti-government demonstrations in Independence Square on December 15. Hide Caption 17 of 77 Photos: Photos: Ukraine protests Ukraine protests – A protester shouts slogans and waves a flag of the Batkivshchyna party during a protest near the Ukrainian Security Service building in Kiev on December 15. Hide Caption 18 of 77 Photos: Photos: Ukraine protests Ukraine protests – Ukrainian opposition supporters gather at a mass rally in Kiev on December 15. Hide Caption 19 of 77 Photos: Photos: Ukraine protests Ukraine protests – Demonstrators by the thousands rally in Independence Square on Saturday, December 14. Hide Caption 20 of 77 Photos: Photos: Ukraine protests Ukraine protests – People go through a police line separating the competing rallies of opposition protesters in Independence Square and supporters of Ukrainian President Viktor Yanukovych in Kiev on December 14. Hide Caption 21 of 77 Photos: Photos: Ukraine protests Ukraine protests – Supporters of the president wave flags of the ruling Party of Regions, as well as Ukrainian flags, during a rally on Kiev's European Square on December 14. Hide Caption 22 of 77 Photos: Photos: Ukraine protests Ukraine protests – Anti-government protesters guard a barricade designed to keep police from evicting them from Independence Square on Friday, December 13. Hide Caption 23 of 77 Photos: Photos: Ukraine protests Ukraine protests – Protesters stand at a barricade in Kiev on December 13. Hide Caption 24 of 77 Photos: Photos: Ukraine protests Ukraine protests – Protesters hold Ukrainian national flags as they picket the Ukrainian House during round-table talks between Yanukovych, former presidents and leaders of the opposition on December 13. Hide Caption 25 of 77 Photos: Photos: Ukraine protests Ukraine protests – Riot police storm barricades set up by pro-EU protesters in Independence Square on Wednesday, December 11. Hide Caption 26 of 77 Photos: Photos: Ukraine protests Ukraine protests – Riot police move against protesters on Independence Square around 2 a.m. December 11. U.S. Secretary of State John Kerry expressed "disgust" over the crackdown. Hide Caption 27 of 77 Photos: Photos: Ukraine protests Ukraine protests – A protester calls for attention inside Kiev's City Hall after riot police were forced out from blocking the front door on December 11. Hide Caption 28 of 77 Photos: Photos: Ukraine protests Ukraine protests – Police enter Independence Square on December 11. Hide Caption 29 of 77 Photos: Photos: Ukraine protests Ukraine protests – Riot police officers block the entrance of Kiev's City Hall as protesters pour water from windows and throw smoke grenades at them on December 11. Hide Caption 30 of 77 Photos: Photos: Ukraine protests Ukraine protests – A young police officer guards a roadblock on Institutskaya Street near the presidential area in Kiev on December 11. Hide Caption 31 of 77 Photos: Photos: Ukraine protests Ukraine protests – Protesters who oppose Yanukovych clash with anti-riot police in Independence Square on December 11. Hide Caption 32 of 77 Photos: Photos: Ukraine protests Ukraine protests – A protester shouts behind riot police standing guard in Independence Square on December 9. Hide Caption 33 of 77 Photos: Photos: Ukraine protests Ukraine protests – A protester in Kiev slams a toppled monument of Vladimir Lenin on December 8. Hide Caption 34 of 77 Photos: Photos: Ukraine protests Ukraine protests – Riot police separate pro-EU protesters and supporters of the ruling party during a large rally held in Independence Square on December 8. Hide Caption 35 of 77 Photos: Photos: Ukraine protests Ukraine protests – Demonstrators shout slogans during the mass rally on December 8. Hide Caption 36 of 77 Photos: Photos: Ukraine protests Ukraine protests – Thousands of protesters gather in Independence Square on December 8. Hide Caption 37 of 77 Photos: Photos: Ukraine protests Ukraine protests – Pro-EU activists shout slogans during the rally on December 8. An estimated 100,000 Ukrainians participated. Hide Caption 38 of 77 Photos: Photos: Ukraine protests Ukraine protests – A young protester shouts slogans near a placard depicting Russian President Vladimir Putin and signed "Fare you well!" during the December 8 rally. Hide Caption 39 of 77 Photos: Photos: Ukraine protests Ukraine protests – Pro-EU activists gather around a huge poster of Yulia Tymoshenko, the jailed former Ukrainian Prime Minister, on December 8. Hide Caption 40 of 77 Photos: Photos: Ukraine protests Ukraine protests – Riot police block protesters during the December 8 rally. Hide Caption 41 of 77 Photos: Photos: Ukraine protests Ukraine protests – A rose, the symbol of the revolution, lies on barricades being built by Pro-EU activists next to the Ukrainian Government building in Kiev on December 8. Hide Caption 42 of 77 Photos: Photos: Ukraine protests Ukraine protests – A pro-EU activist holds a cut-out paper heart as she faces police at the presidential office in Kiev on December 8. Hide Caption 43 of 77 Photos: Photos: Ukraine protests Ukraine protests – EU flags and Ukrainian national flags fly above a crowd of pro-EU activists in Independence Square on December 8. Hide Caption 44 of 77 Photos: Photos: Ukraine protests Ukraine protests – Pro-EU activists shout slogans during the rally in Independence Square on December 8. Hide Caption 45 of 77 Photos: Photos: Ukraine protests Ukraine protests – Demonstrators gather in Independence Square during the mass rally December 8. Hide Caption 46 of 77 Photos: Photos: Ukraine protests Ukraine protests – A pro-EU activist offers flowers to police officers at the presidential office in Kiev on December 8. Hide Caption 47 of 77 Photos: Photos: Ukraine protests Ukraine protests – Anti-government protesters camp in Independence Square early in the morning on December 8. Hide Caption 48 of 77 Photos: Photos: Ukraine protests Ukraine protests – Protesters on Independence Square demonstrate against Yanukovych and call for a closer relationship to the EU on December 7. Hide Caption 49 of 77 Photos: Photos: Ukraine protests Ukraine protests – Protesters prepare food at a camp in Independence Square on December 7. Hide Caption 50 of 77 Photos: Photos: Ukraine protests Ukraine protests – Pro-EU activists attend a rally in Independence Square on December 7. Hide Caption 51 of 77 Photos: Photos: Ukraine protests Ukraine protests – Orthodox believers, carrying icons and crosses, walk during a religious procession outside the parliament building in Kiev on December 6. Hide Caption 52 of 77 Photos: Photos: Ukraine protests Ukraine protests – Police stand guard opposite a sea of protesters near the Ukrainian parliament in Kiev on December 3. Hide Caption 53 of 77 Photos: Photos: Ukraine protests Ukraine protests – Protesters chant slogans outside the parliament in Kiev on December 3. Hide Caption 54 of 77 Photos: Photos: Ukraine protests Ukraine protests – Protesters gather in front of the parliament on December 3. Hide Caption 55 of 77 Photos: Photos: Ukraine protests Ukraine protests – Police stand guard outside the parliament on December 3. Hide Caption 56 of 77 Photos: Photos: Ukraine protests Ukraine protests – Protesters use a bulldozer during clashes with police at the presidential office in Kiev on December 1. Hide Caption 57 of 77 Photos: Photos: Ukraine protests Ukraine protests – A protester throws stones toward riot police on December 1. The crowd chanted "Revolution" and "Down with the Gang" as it gathered in Independence Square. Hide Caption 58 of 77 Photos: Photos: Ukraine protests Ukraine protests – A bleeding protester shouts at a police medic after police pushed protesters off the street leading to the presidential administration building on December 1. Hide Caption 59 of 77 Photos: Photos: Ukraine protests Ukraine protests – Protesters clash with police guarding the presidential administration building on December 1. Hide Caption 60 of 77 Photos: Photos: Ukraine protests Ukraine protests – A Ukrainian protester throws stones at riot police during clashes outside the president's office on December 1. Hide Caption 61 of 77 Photos: Photos: Ukraine protests Ukraine protests – Police and protesters look at an injured person near the presidential office on December 1. Hide Caption 62 of 77 Photos: Photos: Ukraine protests Ukraine protests – Pro-EU demonstrators clash with police near the presidential administration office on December 1. Hide Caption 63 of 77 Photos: Photos: Ukraine protests Ukraine protests – Demonstrators gather in downtown Kiev on December 1. Hide Caption 64 of 77 Photos: Photos: Ukraine protests Ukraine protests – Anti-government protesters gather near Shevchenko University on December 1 in Kiev. Hide Caption 65 of 77 Photos: Photos: Ukraine protests Ukraine protests – A protester injured in clashes with police stands on Independence Square on November 30. Hide Caption 66 of 77 Photos: Photos: Ukraine protests Ukraine protests – Police officers guard Independence Square on November 30 after dispersing a rally. Hide Caption 67 of 77 Photos: Photos: Ukraine protests Ukraine protests – Dozens of protesters were wounded in a clash with police in Kiev on November 30. Hide Caption 68 of 77 Photos: Photos: Ukraine protests Ukraine protests – Ukrainian riot police detain a protester on November 30. Hide Caption 69 of 77 Photos: Photos: Ukraine protests Ukraine protests – Opposition supporters hold EU flags November 30 as they guard the gates of the Mikhailovsky monastery. Hide Caption 70 of 77 Photos: Photos: Ukraine protests Ukraine protests – A woman cries during a rally on November 30. Hide Caption 71 of 77 Photos: Photos: Ukraine protests Ukraine protests – Injured protesters receive medical help in an ambulance after riot police broke up a rally on November 30. Hide Caption 72 of 77 Photos: Photos: Ukraine protests Ukraine protests – Protesters gather over bonfires November 30. Hide Caption 73 of 77 Photos: Photos: Ukraine protests Ukraine protests – Opposition supporters shout slogans and wave flags on November 29. Hide Caption 74 of 77 Photos: Photos: Ukraine protests Ukraine protests – A demonstrator holds a torn portrait of Yanukovych on November 29. Hide Caption 75 of 77 Photos: Photos: Ukraine protests Ukraine protests – Demonstrators hold hands during a rally on November 29. Hide Caption 76 of 77 Photos: Photos: Ukraine protests Ukraine protests – Demonstrators take to the streets in the center of Kiev on November 28. Hide Caption 77 of 77

Ukrainian prosecutors are investigating some top government officials regarding the beating of students at anti-government protests two weeks ago.

The subjects of the criminal investigation include Ukrainian Deputy Secretary for National Security Vladimir Sivkovich, Kiev Police Chief Valery Koryak and Kiev Mayor Oleksandr Popov. A fourth person also is under investigation.

The case will be put before a judge on Monday, who could order the those being investigated to house arrest, said Valery Vilkova, spokeswoman for the general prosecutor.

The tumult in Ukraine goes to the heart of its future ties with Russia and the rest of Europe.

Protests have unfolded in Ukraine since November 21 after President Viktor Yanukovych changed his stance on a trade pact with the European Union that had been years in the making.

He refused to sign the agreement, which would have strengthened cooperation with the European Union. Instead, he opted for closer ties with neighboring Russia.

Hundreds of thousands of Ukrainians have taken to the streets of Kiev to demand that the EU deal be signed

The demonstrations are the biggest the country has seen since the Orange Revolution overturned the results of the country's 2004 presidential election. Protesters have created camps and barricades throughout the city center, and have set up makeshift soup kitchens, said CNN iReporter Harvey Carrol.

The epicenter of the protest has been in the city's central Maidan Square, where protesters have created a giant "revolutionary" Christmas tree decorated with flags and a giant poster of jailed former Prime Minister Yulia Tymoshenko.

On Saturday, a pro-government rally was held at nearby European Square, according to the state news agency Ukrinform. Kiev police estimated about 57,000 people participated in the rally. Ukrainian Prime Minister Mykola Azarov spoke at the gathering.