Demonstrators scuffle with police and smash shop windows in Washington ahead of Donald Trump's inauguration as U.S. president.

Police are out in force in the US, following the inauguration of President Donald Trump, and have now reportedly arrested more than 200 protesters.

Some of the protests seemed to turn into warzones, as riot police fired stun grenades and tear gas at protesters throwing rocks and smashing store and car windows.

A riot squad, wearing helmets and armed with batons and shields, charged at protesters in downtown Washington, with some activists knocked over. Journalists also reported being pepper sprayed and thrown to the ground by police as they covered the protest.

ADREES LATIF/REUTERS Activists race after being hit by a stun grenade in Washington, DC.

Footage aired by CNN showed smoke billowing and the sounds of explosions as riot squads, carrying shields, fired stun grenades.

A small group of protesters were continuing to throw "large pieces of concrete" at police and creating a frantic situation, with people running and knocking others over, a CNN correspondent said.

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Earlier, about 500 people, some wearing masks, marched through the city's downtown, using hammers to claw up chunks of pavement to smash the windows of a Bank of America branch and a McDonald's outlet, all symbols of the American capitalist system.

The crowd chanted anti-Trump slogans and carried signs with slogans including "Make Racists Afraid Again", a play on the New York businessman-turned-politician's "Make America Great Again" campaign slogan, as Trump was sworn in as president.

Washington's Metropolitan Police Department said in a statement that numerous people had been arrested and charged with rioting.

ADREES LATIF/REUTERS Protesters clash with police.

Two police officers sustained minor injuries from people who were trying to avoid arrest, they said.

PROTESTERS RALLY AGAINST TRUMP

"The message I want to send is that Trump does not represent this country. He represents the corporate interests," said Jessica Reznicek, a 35-year-old Roman Catholic aid worker from Des Moines, Iowa, who was part of the protest but did not participate in the violence.

RUPTLY Anti-Trump protesters chained themselves together and set fire to Trump and Pence signs as they closed off access to a checkpoint for the inauguration of president-elect Donald Trump along C Street in Washington DC.

Not far from the White House, protesters scuffled with police, at one point throwing aluminum chairs at an outdoor café.

Bob Hrifko, a member of the "Bikers for Trump" group in town to celebrate the inauguration, was struck in the face when he tried to intervene.

"I know, law and order and all that. We need more order. This ain't right," said Hrifko, who was bleeding from a cut under his eye.

ADREES LATIF/REUTERS A man is treated after being hit by pepper spray.

INAUGURATION CROWD

The number of people who turned out to view the midday swearing-in, on a gray day threatened by rain, appeared to be significantly smaller than the estimated two million who turned out for now-former President Barack Obama's first inauguration in 2009.

Overhead video of the National Mall showed sections of the white matting laid down to protect the grass were largely empty.

ADREES LATIF/REUTERS Protesters demonstrating against US President Donald Trump raise their hands as they are surrounded by police.

More people were expected to be on hand when Trump and his entourage travel along Pennsylvania Avenue to the White House later Friday afternoon.

Earlier, liberal activists with a group called Disrupt J20 intermittently blocked multiple security checkpoints leading to the largest public viewing area for the inauguration. Several were led away by police.

Disrupt J20 protest organiser Alli McCracken, 28, of Washington, said the group was voicing its displeasure over Trump's controversial comments about women, illegal immigrants and Muslims.

RUPTLY One of Donald Trump's supporters was seen charging head first into a group of anti-Trump protesters at a security screening checkpoint in Washington DC, on Friday.

"We have a lot of people of diverse backgrounds who are against US imperialism and we feel Trump will continue that legacy," McCracken said.

SYMPATHY RALLIES ACROSS THE WORLD

Ahead of the inauguration, tensions were high on the streets of Washington, with occasional scuffles breaking out.

ADREES LATIF/REUTERS A police officer falls to the ground as another shoots pepper spray at protesters.

Trump supporters Chris and Karen Korthaus, who carried a life-size cardboard cutout of the former reality TV star, crossed paths with an anti-Trump crowd.

"A protester came over and ripped off the Don's head," Karen Korthaus said as she showed a reporter a video of the incident. "We ran to a pizza shop and taped his head back on".

Carl Beams, 36, from Howell, New Jersey, stood in line with thousands of other Trump supporters waiting to enter the National Mall to view the inauguration.

ADREES LATIF/REUTERS A police officer tries to tackle a protester demonstrating against US President Donald Trump in Washington, DC.

"This is a great moment in history. I wanted to be able to say I was here firsthand," said Beams, who runs a martial arts school.

People reacted across the world.

In Tokyo, several hundred people, most of them expatriate Americans, protested against Trump.

BRYAN WOOLSTON/REUTERS A protester is detained by police in Washington, DC.

In London, activists draped a banner across the British capital's iconic Tower Bridge reading "Build bridges not walls," a reference to Trump's promise to wall off the US-Mexico border.

But in Moscow, Russians hoping Trump will usher in a new era of detente with their country celebrated his inauguration.

Russian nationalists held an all-night party at what used to be the main Soviet-era post office in Moscow.

ADREES LATIF/REUTERS An activist is helped after being hit by pepper spray.

East of Moscow, in the city of Zlatoust, craftsmen released a limited series of silver and gold commemorative coins, engraved with "In Trump We Trust."

The US Secret Service, Washington police and other law enforcement agencies had about 28,000 officers in place to secure a roughly 7.8 square km of downtown Washington.

Along the stretch of Washington where the rioters smashed windows, workers were beginning to clean up the debris.

BRYAN WOOLSTON/REUTERS Protesters chain themselves to an entry point prior at the inauguration of US President-elect Donald Trump in Washington, DC.

"We're just working, and the next thing you know, violence is coming our way," said Edwin Garcia, 26, a line cook at an Au Bon Pain where three windows were shattered.

"What was the point if they never got to where Trump is?"

AFP Security forces are in place at dawn on the morning of Donald Trump's inauguration. Trump supporters and anti-Trump demonstrators gather on Pennsylvania Avenue.

ITN Anti-Trump campaigners have dropped a giant banner on Tower Bridge in London, urging Donald Trump to 'Build Bridges Not Walls'.