New clashes in Cairo's Tahrir Square

media captionBBC's Lyse Doucet in Tahrir Square: "The anger... has now boiled over"

Clashes have again erupted in the Egyptian capital as security forces continue their efforts to clear Cairo's Tahrir Square of protesters.

Protesters fear the interim military government is trying to retain its grip on power ahead of parliamentary elections planned to begin next week.

One minister has resigned in protest at the handling of the latest unrest.

At least 20 people are reported to have died since the violence began on Saturday with hundreds more injured.

Earlier reports said 33 people had died, but mortuary officials later corrected the figure, saying some of the deaths had not been related to the protests.

Some 1,750 people have also been reported injured in clashes across the country.

Culture Minister Emad Abu Ghazi has resigned and 25 Egyptian political parties also called for the ministers of information and the interior to be sacked over the violence.

A group of senior Egyptian diplomats have also issued a statement condemning the way the protests have been handled, reports the BBC's Jon Leyne in Cairo.

The statement, signed by more than 109 ambassadors and other diplomats, calls for violence and aggression by the security forces against protesters to be halted immediately, and for those responsible to be brought to justice.

The statement says a full hand-over of power to a civilian government should be completed by the middle of next year.

The Supreme Council of the Armed Forces, led by Field Marshal Mohamed Tantawi, is charged with overseeing the country's transition to democracy after three decades of autocratic rule under ousted President Hosni Mubarak.

Calls for Field Marshal Tantawi's resignation could be heard during the weekend's protests.

It is the longest continuous protest since President Mubarak stepped down in February and casts a shadow over elections due to start next week.

Large crowds again streamed into Tahrir Square on Monday - defying the military's attempts to keep them away from the place that was the symbolic heart of demonstrations against Mr Mubarak.

TV footage showed tear gas being fired into the protesters, while fire bombs and chunks of concrete were reportedly being lobbed back at the police.

The BBC's Lyse Doucet in Cairo tweeted that medical students joined the protest on Monday with a banner calling for power to be handed over by April 2012.

As daylight faded, even more people were filling Tahrir Square, she added.

The clashes followed fierce fighting on Sunday. Violence also took place in other cities over the weekend, including Alexandria, Suez and Aswan.

Fresh demands

Amr Moussa, former secretary-general of the Arab League and now a presidential candidate in Egypt, told the BBC World Service that the use of force against the protesters could not be justified.

"The way the police deals with the protesters... we're all against this kind of violence and this treatment of the people," he said.

He said the military council needed to end the uncertainty surrounding parliamentary and presidential elections.

Earlier, Culture Minister Emad Abu Ghazi resigned in protest at the government's handling of events in Tahrir Square, Egypt's official Mena news agency said.

The BBC's Yolande Knell in Cairo says the demands of the protesters have changed over the course of the weekend. Crowds initially urged the military to set a date for the handover of power, but now they want the military leaders to resign immediately.

"The military promised that they would hand over power within six months," one protester said. "Now 10 months have gone by and they still haven't done it. We feel deceived."

In recent weeks, protesters - mostly Islamists and young activists - have been demonstrating against a draft constitution they say would allow the military to retain too much power after a civilian government is elected.

Earlier this month the military council produced a draft document setting out principles for a new constitution, under which the military and its budget could be exempted from civilian oversight.

A proposal by the military to delay the presidential election until late 2012 or early 2013 has further angered the opposition.

Protesters want the presidential vote to take place after parliamentary elections, which begin on 28 November and will be staggered over the next three months.

A statement from the cabinet on Sunday said the elections would go ahead as planned, and praised the "restraint" of interior ministry forces against protesters.

The military council, in a statement read out on state television, said it "regretted" what was happening, AFP news agency reports.