Egyptian activists have called for mass demonstrations after an overnight stand-off between security forces and protesters in Cairo's Tahrir Square. Thousands flocked to the square after the military-appointed cabinet offered its resignation. At least 26 people have been killed and hundreds injured in three days of protests against the military rulers. Amnesty International has criticised the military for failing to keep its promise to improve human rights. Late on Monday some 20,000 people were estimated to have thronged Tahrir Square after the cabinet of Prime Minister Essam Sharaf announced its offer to resign. Their numbers were far short of the hundreds of thousands whose demonstrations in the square earlier in the year led to the overthrow of Hosni Mubarak as president after 30 years in power. Tear gas and bullets But the square was again the scene of clashes between security forces and protesters determined to see the military transfer power to a civilian government. A coalition of political groups has accused the military council of leading a "counter-revolution" and called for a mass demonstration on Tuesday. However, the Muslim Brotherhood - whose Freedom and Equality party is widely seen as the most formidable contender in next week's elections - said it will not take part in the protests. This is likely to be because they want the elections to go ahead in order to show much support they have got in the country, the BBC's Jon Leyne in Cairo reports. Hundreds of people camped out in the square overnight. The Supreme Council of the Armed Forces has not said whether it has accepted the cabinet's resignation. A military source told the BBC there was no consensus among the council on whether to accept. The source said the council was also consulting with other political groups. Many people had already written the cabinet off as a spent force, as the ruling military council has assumed more and more direct control of the running of the country, our correspondent says. In a statement carried by the official Mena news agency, the council expressed its "deep regret for the victims in these painful incidents" and called for "the greatest degree of self-restraint". It said it had asked the justice ministry to form a committee to investigate the violence of the last few days. On Monday evening, ambulances with sirens wailing were seen driving into the heart of the large city-centre square to ferry the injured to hospital. During the day, there were running battles on the edges of the square and the nearby sidestreets leading to the interior ministry. Protesters threw stones at the riot police and troops who used their batons and fired tear gas and birdshot. Medics told the BBC they were seeing people injured by tear gas and rubber bullets. There was violence in other parts of Egypt too. Two people died in the port city of Ismailia on the Suez Canal after security forces clashed with some 4,000 demonstrators, witnesses said. The clashes began on Saturday following demonstrations against proposed constitutional changes unveiled by the interim government. Protesters were chanting "the people want the removal of the marshal" - a reference to Field Marshal Mohamed Tantawi, who heads the Supreme Council of the Armed Forces and was Hosni Mubarak's defence minister for 20 years. He has the task of overseeing the country's transition to democracy after three decades of autocratic rule under Mr Mubarak. 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. The unrest has cast uncertainty over the future of the elections but the military council has said they will go ahead. Many people fear the military plans to hold on to the reins of power, whatever the outcome. The cabinet's resignation move poses a dilemma for the military, our correspondent says. Any future cabinet is bound to demand more power for the civilian government. Or, he says, the military will be forced to take more direct control, making the generals personally responsible for all that is now going wrong in Egypt. In a report published on Tuesday, Amnesty International said Egypt's military rulers had "completely failed to live up their promises to Egyptians to improve human rights". Amnesty said the military council had carried on many of the abusive tactics of the Hosni Mubarak era, including torture of suspects, targeting critics and banning critical media. "The euphoria of the uprising has been replaced by fears that one repressive rule has simply been replaced with another," Amnesty said.