CAIRO (AP) - The leadership of Egypt's ruling party stepped down Saturday as the military figures spearheading the transition tried to placate protesters without giving them the one resignation they demand, President Hosni Mubarak's. The United States gave key backing to the regime's gradual changes, warning of the dangers if Mubarak goes too quickly.

But protesters in the streets rejected the new concessions and vowed to keep up their campaign until the 82-year-old president steps down. Many are convinced that the regime wants to wear down their movement and enact only superficial democratic reforms that will leave its deeply entrenched monopoly on power in place.

Tens of thousands thronged Cairo's central Tahrir Square in a 12th day of protests, waving flags and chanting, "He will go! He will go!"

Mubarak, who has ruled Egypt with an authoritarian hand for nearly 30 years, insists he must stay in office until his term ends, after a September presidential election. The military figures he has installed to lead the government - Vice President Omar Suleiman and Prime Minister Ahmed Shafiq - have offered in the meantime to hold negotiations with the protesters and the entire opposition over democratic reforms to ensure a fair vote.

A day after President Barack Obama pushed Mubarak to leave quickly, the U.S. administration changed tone Saturday with a strong endorsement of Suleiman's plans.

"It's important to support the transition process announced by the Egyptian government actually headed by now-Vice President Omar Suleiman," Secretary of State Hillary Rodham Clinton said at an international security conference in Munich, Germany. She warned that without orderly change, extremists could derail the process.

A U.S. envoy who met Mubarak earlier this week, former ambassador Frank Wisner, went further still, saying it is "crucial" that Mubarak remain in place for the time being to ensure reforms go through. He pointed out that under the constitution, a Mubarak resignation would require new elections in two months, meaning they would take place under the current rules that all but guarantee a ruling party victory.

His comment was an abrupt change in message - on Friday, Obama called on Mubarak to "make the right decision." The State Department later said Wisner was speaking as a private citizen since his official mission to Egypt had ended.

America's confidence in Suleiman is not shared by the protesters, who doubt the ruling party will bring democracy unless they continue their mass demonstrations.

They want the concrete victory of Mubarak's removal with a broadbased transitional government to work out a new constitution.

"What happened so far does not qualify as reform," said Amr Hamzawy, a member of the Committee of Wise Men, a self-appointed group of prominent figures from Egypt's elite that is unconnected to the protesters but has met with Suleiman to explore solutions to the crisis. "There seems to be a deliberate attempt by the regime to distract the proponents of change and allow the demands to disintegrate in the hope of (regime) survival."

That could mean the crisis could move into a test of sheer endurance, as protesters try to keep drumming out tens of thousands into Tahrir day after day.

The government and military have promised not to try to clear protesters from the square, and soldiers guarding the square continued to let people enter to join the growing rally.