Egypt's security chief fired nearly 700 police officers in a step to cleanse the much-hated force, the latest concession military rulers have made under pressure from protesters holding a sit-in in Cairo's Tahrir Square for the past six days.

Widespread abuses by the police under the former regime were a key reason behind the protests that toppled President Hosni Mubarak in February. But the ruling military council that took over from him has been slow to hold ex-regime officials and police accountable for killing nearly 900 protesters during the uprising and other crimes.

With public frustration rising sharply, protesters resumed a sit-in in Tahrir Square, the epicenter of the revolution that was occupied day and night for most of the 18-day uprising. Protesters say the dismissal of 669 police officers was not extensive enough.

"These are just sedatives. We won't be fooled," said Walid Saoud, a 34-year-old protester among the crowd in Tahrir on Wednesday. He said the sit-in will go on because the protesters want to see a total restructuring of the police force, the main tool of political control under the previous regime.

Some are even accusing the ruling military council of trying to protect Mubarak and his former regime loyalists.

In another nod to demands by activists, the military is delaying parliamentary elections that had been expected in September, the state news agency said. The vote is now expected in October or November, the report said.

Many of the political parties that arose from the uprising want the delay so they can compete more effectively against better prepared and financed Islamist groups, such as the Muslim Brotherhood.

Despite the latest string of concessions, protesters appear more determined than ever to press their demands. Many are calling for the dismissal of the current Cabinet.

"Tahrir Square is the popular and legitimate monitor of the performance of any institution in Egypt," Saoud said. "As we see it, there has been no change in the police ways."

Interior Minister Mansour el-Issawi on Wednesday announced the dismissal of 669 high-ranking police officers including 505 major-generals _ 10 of them top assistants to the minister. State TV said 37 of the dismissed officers face charges of killing protesters.

"This is the biggest administrative move ... to bring new blood," to the police force, el-Issawi said. He promised that "any police officer will be held accountable for any violation."

How to deal with Egypt's delegitimized police force, with nearly half a million members, has been a major testing ground for the interim government.

During the early days of the uprising, there were intensely violent clashes when police used force to try to hold back peaceful marches. A few days into the uprising, the police melted away from the streets mysteriously, leaving the streets to waves of looting and theft.

The force has not redeployed in full force since, and some blame police for the rising wave of crime in Egypt in the past months.

Activists say lack of security is the biggest challenge to meeting their demands to overhaul the system, including holding fair and free elections.

Magda Boutros, an activist and part of a team that launched a new initiative to reform the police force, said the measures introduced by el-Issawi fall short of expectations.

She said her group estimates that at least 200 officers responsible for the killing of protesters and facing trials should have been dismissed.

"We can't take one of Mubarak's biggest problems and deal with it piecemeal. We need a comprehensive plan," she said.

Mohammed Abdel-Aziz, a lawyer specializing in torture cases, said torture continued after the revolution. He is currently representing three who were abused by a police officer. Abdel-Aziz said the minister's plan didn't send a "deterrent" message to middle level officers.

Many fear a total shake-up of the ministry may lead to revenge from the ousted force, or a widening of chaos because of a leadership gap.

Ahmed Ragab, spokesman for a pro-reform association of police officers, said restructuring has to be gradual and the new leadership must be given time.

Mubarak's former security chief Habib el-Adly and six of his aides are currently on trial to face charges of ordering the killing of protesters. Mubarak himself is facing similar charges and his trial begins next month.

Protesters still in Tahrir lifted their siege of Cairo's largest government building Wednesday, allowing business to resume there while staying camped out in the square to press the military rulers for faster change.

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Associated Press writer Maggie Michael contributed to this report.