Riot police and government supporters beat dozens of pro-reform activists with clubs, sometimes kicking them as they lay on the ground Saturday during a protest in the Egyptian capital against President Hosni Mubarak (search).

Opposition groups were gathering to stage a demonstration in Cairo's main Liberation Square (search) two days after Mubarak announced he would run for re-election in a Sept. 7 vote.

The election will be Mubarak's first in which he faces an opponent, and his government has touted it as a launching pad for greater democracy. The United States (search) also praised the elections, though Mubarak opponents are more skeptical.

Riot police blocked protesters from reaching Liberation Square, and plainclothes officers wielding police batons quickly descended on them and beat them.

The scuffles lasted for about an hour, and an Associated Press reporter saw least 15 protesters being beaten.

In several places, groups of five or six men surrounded a single protester on the ground, kicking and hitting the person with batons.

Dazed and bruised protesters were thrown into nearby police vans. Black-uniformed police also joined in the assaults.

Police and government supporters chased after fleeing men and women. Men in plainclothes lifted some protesters off the ground and hauled them into trucks.

Some activists lay down briefly in front of a police van full of detained protesters but got out of the way when it began moving.

An elderly man wandered, dazed, with blood running from his scalp, while a young man staggered away with torn clothes. Other protesters wept.

Several hundred protesters appeared to be at the scene, chanting slogans against Mubarak, but precise numbers were difficult to gauge because the protesters were immediately assaulted.

The clashes came in the wake of similar violence in May during a constitutional referendum, when government supporters attacked and sexually assaulted women at a reform protest. That violence drew criticism from the United States, which has been pressing Egypt, a close ally, to ensure the September presidential election is fair and democratic.

During a June 6 visit to Egypt to press for reform, Secretary of State Condoleezza Rice condemned the referendum assaults as a "sad incident from the point of view of Egyptian reform," saying the government must "make certain that people can associate and can peacefully petition."

Police arrested several top figures from the Kifaya movement, one of the most vocal anti-Mubarak organizations, said Aida Seif al-Daula, a prominent activist. She said she had been beaten on the head with a stick but was not seriously injured.

Police tried to arrest Wail Khalil, another activist, but several colleagues managed to wrestle him free.

"They were dragging me ... They arrested a lot of people today. They hit the protest before it even started," Khalil told AP afterward, wearing an orange T-shirt drenched in sweat.

"It's very clear that the orders today are slaughter," he said.

Just as he said this, several men, including one in a white T-shirt carrying a gun, surrounded Khalil and took him away.

Most major opposition groups are boycotting the September election, calling Mubarak's move to open the vote to multiple candidates a sham that will not dent his 24-year rule. The 77-year-old Mubarak is expected to easily win the vote.