(CNN) -- Security forces in restive Yemen fired live ammunition during protests Saturday in Change Square outside Sanaa University, witnesses claimed.

But a government source knocked down those accounts, asserting that the police used only water cannon and tear gas to disperse crowds, and that third-party provocateurs fired at the people.

This was one of at least five cities where protesters assembled to demand the ouster of President Ali Abdullah Saleh, who has ruled Yemen since 1978.

One person died from a gunshot wound to the head at the square in Sanaa, according to Waseem Al-Qurashi, a doctor and head of the field medical team that was set up outside the college. Opposition medical officials in Mukalla said a protester was shot and killed there.

Human Rights Watch said at least two people were killed and 100 wounded in the pre-dawn attack near the entrance to Sanaa University.

Casualties during protests this week drew stiff criticism from the United Nations and the United States Saturday.

The United Nations released a statement in which Secretary-General Ban Ki-moon called for restraint and urged the government and opposition groups to begin serious talks.

"In particular, he (Ban) condemns the excessive use of force by the security forces of the Government of Yemen against peaceful demonstrators in Sanaa on 12 March, which has led to the death of several people and the injury of many others," it read.

The United Nations said the government of Yemen must "investigate the allegations of extrajudicial killings and human rights violations."

The U.S. Embassy in Yemen similarly called on the government to investigate.

"The embassy is dismayed by reports of deaths and injuries at demonstrations in Sanaa, Aden, Taiz, and Mukalla in the past week," it said in a statement.

In the capital, Sanaa, Abdullah Alansi, an anti-government protester, said security forces approached protesters soon after morning prayers.

"They told the protesters that they needed to remove their tents from the grounds and leave. The protesters refused. After that the security forces started attacking the demonstrators," Alansi said.

"About an hour after the security forces first shot live ammo and then threw gas at us, protesters started fighting back -- they started throwing rocks at the security forces," Alansi said. "Then the security forces retreated for about 15 or 20 minutes and then started attacking again."

Anti-government demonstrator Ala'a Al-Khowlani said more people came out to support the protesters after the attacks.

Al-Khowlani also said students were donating blood at field hospitals that have been set up, as wounded protesters are in "dire need of blood."

Sameer Sameie, a medic working in one of the field hospital tents set up at the square, said security forces used tear gas on protesters, entered the crowd and beat some demonstrators. He said officers entered the tent where he was working and roughed him up, along with others.

Sameie called for international aid and rights organizations for assistance because medics aren't able to properly treat the injured.

But a Yemeni Interior Ministry official, who asked not to be named because he was not authorized to comment, said riot police were under strict orders not to carry live ammunition and used tear gas and water cannons to disperse the crowd. He said third-party instigators fired at the protesters.

More than 160 police, demonstrators, residents and bystanders were injured by bullet wounds, physical attacks and rocks, the official said.

He said residents in various neighborhoods complained to police that protesters forcefully erected additional tents in front of homes and stores. They said they felt surrounded and besieged and women were being harassed.

Residents warned that if police didn't confront the demonstrators, they would push back themselves.

Abdulkarim al-Aslami, a parliament member who resigned from the ruling party and joined the opposition, said the scene in the square "is very dangerous," and he saw many demonstrators beaten and gassed. He said lawmakers have appointed a committee to investigate what happened on Saturday.

"Things are taking a very dangerous turn here. The regime ended any chance of dialogue."

Ahmad Sufan, a former prime minister and parliament member with good ties to the president, said he doubted that police were behind the violence and unrest in the square and called for dialogue.

"We don't really know who to blame. If the security forces are responsible then they should be punished," said Sufan, who said it is unfortunate that political debate is taking place in the streets and not in government institutions.

Along with Mukalla and Sanaa, demonstrators took to the streets in Taiz, Maafir, and Mawasit.

In Maafir, witnesses said protesters raided government and security buildings and set vehicles on fire.

A medical team said the forces fired tear gas and fired randomly at protesters, with 34 injuries reported. Witnesses said more than six people were injured by security forces and four suffered gunshot wounds.

Across Yemen, high unemployment fuels much of the anger among a growing young population steeped in poverty. The protesters also cite government corruption and a lack of political freedom.

On Thursday, Saleh -- who has promised not to run for president in the next round of elections -- pledged to bring a new constitution to a vote by the end of the year and transfer government power to an elected parliamentary system.

John Brennan, President Barack Obama's counterterrorism adviser, spoke with Saleh on Friday and welcomed his "initiative to resolve the political crisis in Yemen," according to a statement from the White House.

Brennan also said the government and opposition "share responsibility for achieving a peaceful resolution of the crisis."

Saleh said he hopes the opposition will engage immediately in a dialogue with the government and reiterated his assurance to not use violence against peaceful protesters, according to a summary of Brennan's call with Saleh.

Human Rights Watch released a statement Saturday, calling on countries that support Saleh to make aid contingent on the government stopping its attacks on demonstrators.

"President Saleh has once again broken his promise to end attacks on peaceful protesters, and those who supply his government with weapons risk complicity in this bloodshed," said Joe Stork, deputy Middle East and North Africa director at the rights organization.