CAIRO – Egyptian President Hosni Mubarak came under fresh pressure on Monday to step down as opponents said concessions made in landmark talks were not enough to halt a revolt against his 30-year rule.

Thousands of demonstrators spent Sunday night under blankets and tarpaulins in central Cairo's Tahrir Square, or Liberation Square, which over two weeks has begun to resemble a tented camp.

Protesters sat under the tracks of army tanks deployed around the square, fearful that any military pull-out could be designed to drive out the protesters or abandon them to the mercy of pro-regime thugs.

Activists barred access to the hulking Mugamma building, the heart of Egypt's bureaucracy, which dominates the square, despite dozens of people seeking to have documents such as passports or birth certificates processed.

Protesters captured a man with a petrol can they said was trying to set the building ablaze, an act of arson that would be blamed on protesters, and handed him over to troops.

Mubarak's key lieutenant and possible heir, Vice President Omar Suleiman, attempted on Sunday to appease the revolt by inviting some opposition groups to join him on a panel to pilot democratic reform.

But the demonstrators, in their 14th straight day of protest on Monday, were unimpressed.

Opposition parties, including the powerful Muslim Brotherhood, repeated their demand that Mubarak himself must stand down or immediately delegate his powers to his deputy.

And there was scant relief for Mubarak in the Western capitals where he was once hailed as a close ally and bulwark of Middle East stability.

US President Barack Obama says Egypt has changed forever since its street revolt broke out on January 25 and has called for a "representative government" in Cairo although he stopped short of urging Mubarak to quit immediately.

"He's not running for re-election. His term is up this year," Obama said.

Government spokesman Magdi Radi said the parties had agreed to form a committee of jurists and politicians "to study and propose constitutional amendments and required legislative amendments ... by the first week of March."

Negotiators also agreed to open an office for complaints about the treatment of political prisoners, to loosen media curbs, lift emergency rule "depending on the security situation," and reject foreign interference.

But Suleiman refused another key demand of the opposition, saying he would not assume Mubarak's powers and rule in his place during the transition.

Not all of the opposition movements involved in the revolt against Mubarak's rule were present at the talks. Former UN nuclear watchdog head and leading dissident Mohamed ElBaradei was not invited, and has criticised the talks.

The Muslim Brotherhood, still officially banned, said it had agreed to take part in the talks because it wanted to determine if the government was serious about reform, but warned that the initial concessions were insufficient.

Asked whether he believed Mubarak would step down, the powerful Islamist movement's number two leader Mahmud Ezzat told AFP: "That hinges on popular pressure, and we support the popular pressure. It must continue."

Mubarak has thus far refused demands to step down immediately.

While he has said he is "fed up" with leadership, Mubarak says he feels he must stay on until September's presidential election in order to ensure stability -- but the demonstrators' frustration is now finding an echo abroad.

Spain's foreign minister said the election should be brought forward.

Egypt "could find a way to answer the legitimate aspirations of the citizens if the authorities made a gesture and brought forward to the month of June the elections," Trinidad Jimenez told Spain's El Mundo newspaper.

US Secretary of State Hillary Clinton said the timing of Mubarak's exit depended on the Egyptian people but warned that an early date could lead to complications if opposition groups are not organised for the vote.

"As I understand the constitution, if the president were to resign, he would be succeeded by the speaker of the house, and presidential elections would have to be held in 60 days," she said.

Clinton said she had heard a Muslim Brotherhood leader as well as ElBaradei say that "it's going to take time" to organise polls.

This cuts little ice in Tahrir Square, however, where the demonstrators have kept up demands for his immediate exit and have no faith that the 82-year-old leader is serious about stepping down after three decades in power.