The Egyptian military guarded thousands of protesters pouring into Cairo's main square today in an attempt to drive out President Hosni Mubarak.

The Obama administration said it was in talks with top Egyptian officials about the possibility of Mubarak immediately resigning, and an interim government forming before free and fair elections this year.

Egyptian Defence Minister Hussein Tantawi and senior army officials visited the square this morning and soldiers were checking IDs and performing body searches at its entrances, a sign that Egypt's most powerful institution was sanctioning the demonstration.

The atmosphere was calm after two days of battles between pro- and anti-Mubarak crowds battling with stones and makeshift shields.

The protester have vowed to intensify efforts to force the president out by today.

The Facebook page that started the protest movement said supporters should gather at noon on all Egyptian squares "so that we can put the last nail in the regime's coffin, and declare the victory of the January 25 revolution".

Gangs backing Mubarak attacked journalists and human rights activists across Cairo yesterday, while others were detained by soldiers.

US officials said the creation of a military-backed caretaker government was one of several ideas being discussed between the Egyptian regime and the Obama administration.

The American officials spoke on condition of anonymity to discuss the continuing sensitive talks.

Among the options was a proposal for Mubarak to resign immediately and cede power to a transitional government run by Vice President Omar Suleiman, the officials said.

This morning, human chains of protesters performed secondary searches inside Tahrir Square.

Many of those arriving brought fresh bread, water, fruit and other supplies, and the atmosphere was relaxed. Long lines formed at tables of people handing out tea and bread.

Around the square were makeshift clinics, set up in the entranceways of stores, including a KFC. At one, a man received an injection in his arm. Above another was the sign of an interlocking crescent and cross.

The pro-Mubarak crowds that have attacked demonstrators and foreign journalists did not have a visible presence.

Ayman Nour, a former presidential candidate who is a member of a new committee formed by various factions to conduct any future negotiations on the protesters' behalf once Mubarak steps down, said that he hopes the demonstration "leads to Mubarak's departure."

"The chaos is organised by his ruling party," Nour said. "There is a fifth column inside the regime that led the looting and violence."

Suleiman said yesterday he had invited the fundamentalist Muslim Brotherhood into negotiations over Egypt's future and the transition to democracy - a stunning concession to a group that the regime considers its worst enemy and has cracked down on ferociously in the past years.

The Brotherhood has rushed to take a stronger role in the unprecedented protests that erupted 10 days ago, led by more secular young activists demanding the removal of Mubarak.

The Brotherhood's strength was on display in the pitched battles on Wednesday and yesterday against government supporters who attacked the protesters' camp in Cairo's central Tahrir Square before they were driven from the square by the pro-democracy forces.

Brothers - distinguishable by their close-cropped beards - dominated the front lines, often lining up to pray for "victory or martyrdom," before throwing themselves into the fray, hurling stones, sticks and firebombs at the attackers while shouting "God is great."

Amr Said, a 41-year-old chemist who said he is a Brotherhood supporter, told the Associated Press in Tahrir Square today that "our instructions are not to assume a role that is too visible at the moment, and to get along with all other groups including and leftist and liberals.

"We also refrain from making our typically brotherhood chants and when one of us does, we quickly shut him up," he said.

The potential of the Brotherhood gaining greater power has clearly weighed on the United States as it presses Mubarak to bow out. US officials have said they want the transition to democracy to be stable to prevent any group from imposing its ideology.

The editor of the Muslim Brotherhood's website told the AP that policemen stormed its office this morning and arrested 10 to 15 of its journalists. Abdel Galil el-Sharnoubi said that the website was also being blocked.

One of the leaders of the protest movement, Nobel prize winner Mohamed ElBaradei said Mubarak should step down now with dignity.

Mubarak "should hear the clear voice coming from the people and leave in dignity."

He dismissed concessions so far by Mubarak, calling them "piecemeal," adding "it's a question of trust, and the trust is gone."

He told reporters there should be a year-long transition to democracy under a temporary constitution with a presidential council of several people, including a military representative.