Hundreds of thousands of demonstrators gather in Tahrir Square to demand immediate exit of Field Marshal Hussein Tantawi

Egypt's revolution has been plunged into fresh uncertainty after hundreds of thousands of angry demonstrators rejected a promise by the country's military council on Tuesday to accelerate the transition to civilian rule.

In an extraordinary display of people power, protesters at a mass rally in Cairo's Tahrir Square demanded the immediate departure of Field Marshal Mohamed Hussein Tantawi, the head of the Supreme Council of the Armed Forces (Scaf), just as they had demanded President Hosni Mubarak's humiliating exit in February.

"We are not leaving, he leaves," the crowd chanted.

Tantawi, who served as Mubarak's defence minister for two decades, appeared on state television in full military uniform to announce that a first round of parliamentary elections would go ahead as planned next week and that presidential elections – seen as crucial to real civilian rule – would be brought forward to next summer.

Previously the military had floated late next year or early 2013 as the date for transferring power.

Tantawi said he was accepting the resignation of the civilian caretaker government led by Essam Sharaf, and that he was sorry for the estimated 30 people who had died in the latest unrest.

Egyptian media reported that Sharaf could be replaced as the head of a new government of national salvation by Mohamed ElBaradei, the former chief UN weapons inspector.

Turnout in Tahrir Square was less than the million people that organisers had hoped for, but it was still a massive display of popular will on a scale that was also the hallmark of the uprising in January that ousted Mubarak. Hours before the rally was due to begin makeshift hospitals around the site were struggling to cope with the injured. Medical sources said 500 people were injured in two hours alone – one every five seconds, Al-Ahram Online reported.

"The armed forces, represented by their supreme council, do not aspire to govern and put the supreme interest of the country above all considerations," Tantawi declared.

The military did not "care about who will win" and "it's up to the people to decide who will rule," he said. The army was "completely ready to hand over responsibility immediately", and to return to its original mission to protect the country if the country wanted that, via a popular referendum if need be.

"Some tried to drag us into confrontation," he said. "But we will control ourselves to the maximum. We will never kill a single Egyptian."

As his broadcast ended, chants of "go, go, the people demand the overthrow of the regime," erupted from the crowd in Tahrir Square. Tantawi, like Mubarak in February, appeared to be far behind popular demands.

Not all reaction was negative. The Muslim Brotherhood, likely to emerge as frontrunner in the parliamentary elections, and anxious they take place on schedule, appeared to indicate that it was satisfied with the amended timetable.

But there was a powerful sense that popular pressure had forced the pace.

Earlier, in an electrifying moment, an army officer left his men to join the protesters while an effigy of Tantawi was hanged to cheers.

Huge crowds also gathered for a fourth consecutive day in Alexandria, Egypt's second city, where dense clouds of tear gas and gunfire could also be seen and heard live on television as uniformed police and plainclothes agents were deployed.

Opposition leaders said after talks with Scaf earlier that the military's position was inadequate. "Our demands are clear. We want the military council to step down and hand over authority to a national salvation government with full authority," said Khaled El-Sayed, a member of the Youth Revolution Coalition and a candidate in the parliamentary election.

The commander of the military police and the interior minister, who is in charge of the police, must be tried for the "horrific crimes" of the past few days, he added.

The pace of events caught western governments on the hop, unsure whether to go beyond demands for an end to the violence, to call for the imminent elections to be postponed, or, more ambitiously, for the Scaf to surrender power.

In Washington, the White House and state department condemned the force used in the last few days. "We are deeply concerned about the violence. The violence is deplorable. We call on all sides to exercise restraint," said White House spokesman Jay Carney.

In London, the Egyptian crisis was discussed at the National Security Council and in talks between David Cameron and Turkish President Abdullah Gul.

Human Rights Watch urged Egypt's military to immediately order riot police to stop using excessive force against protesters. "This latest crisis is a reminder of everything that has not happened in the past months during Egypt's promised transition," it said. "We have yet to see the military begin reforming the security services or ending the abusive practices and policies of the Mubarak era."

Egypt's health ministry said at least 29 people have died in clashes between the security forces and demonstrators since Saturday. Medical sources counted at least 36 dead and more than 1,250 wounded. Three people died early yesterday in the Red Sea port city of Ismailia.