Egypt defended on Friday the presence of thousands of riot police who chased and beat pro-democracy activists during protests the day before, as prosecutors extended the detention of more than 250 people arrested in the demonstrations.

The violence broke out when protesters tried to gather in support of two judges due to appear Thursday at a disciplinary hearing after they blew the whistle on fraud during parliament elections last year.

The Interior Ministry put out a statement saying police forces were deployed outside a Cairo court where the hearing was to be held to "protect this important judicial institution."

The ministry, citing an unnamed security official, said the police presence was necessary because people had "gathered and protested without obtaining a permit."

The statement did not comment on accusations that police beat demonstrators.

Thursday's violence appeared to signal that the Egyptian government, a top U.S. ally, is taking a new stance of zero-tolerance toward protests demanding reform amid complaints that President Hosni Mubarak has backed away from promises for greater democracy.

The two judges — Hisham el-Bastiwisy and Mahmoud Mekki — became heroes of the democracy movement after they went public with claims of fraud during parliament elections last year.

According to the statement, the prosecution was interrogating "elements who mostly belonged to illegal entities and who have insisted on gathering and obstructing traffic in the downtown area," an apparent reference to members of the banned Muslim Brotherhood who were among those arrested on Thursday.

The statement did not provide figures for the arrests, but said those held "failed to respond to repeated warnings to leave and had to be arrested to enforce the law."

CountryWatch: Egypt

A prosecution official on Friday ordered that 255 people arrested during the protest be detained for 15 days for questioning, a judicial source said on condition of anonymity because he was not authorized to give statements to the media.

The director of the Brotherhood's Web site, Abdel Gelil el-Sharnoubi, said Friday that up to 300 Brotherhood members were detained across the country on Thursday in conjunction with their support for the judges. Mohammed Taima of the pro-reform Kifaya movement said about 60 Kifaya members across Egypt were arrested Thursday. About 10 of them were later released, he added

Taima said lawyers informed Kifaya that its arrested members were ordered held for 15 days.

The U.S. State Department said it was "deeply concerned" about the police assault on protesters and would be raising the matter with the government.

"We urge the Egyptian government to permit peaceful demonstrations on behalf of reform and civil liberties," spokesman Sean McCormack has told reporters.