TURMOIL IN EGYPT…. As I get ready to step away from my desk for the afternoon, it occurs to me I can’t say with confidence whether Egypt’s Mubarak government will still exist by the time I get back in a few hours. Indeed, as of this afternoon, there’s apparently a credible presidential alternative.

The Egyptian military reinforced parts of the capital on Sunday with tanks, jets and helicopters as tens of thousands of protesters flooded central Cairo for the sixth day, defying yet again government orders of a nationwide curfew.

The uprising, which began as a spontaneous grass-roots movement, appeared to coalesce, at least for the moment, as the largest opposition group, the Muslim Brotherhood, threw its support behind a leading opposition figure, the Nobel laureate Mohamed ElBaradei, to negotiate on behalf of the protesters.

Mr. ElBaradei arrived in Liberation Square, the center of the protests, shortly after nightfall and addressed the crowd through a bullhorn.

“We are beginning a new era in Egypt,” he said. “What we have begun cannot be reversed.

“We have a key demand: for Mubarak to step down and to start a new era.”

Watching Al Jazeera English, it’s also important to emphasize that the unrest is not limited to Cairo — the protests were massive and boisterous in Alexandria today as well.

In the meantime, the police in the capital effectively gave up today, raising fears of lawlessness, and reinforcing the notion that the government itself was on the verge of collapse.

As for the closely-watched Egyptian army, troops took no action against protestors today.

U.S. officials, meanwhile, have not called for Mubarak’s ouster, though Secretary of State Hillary Rodham Clinton this morning called for “an orderly transition” in Egypt to a system that can “meet the democratic and economic needs of the people.” She added that Egypt needs a “real democracy.”

If Mubarak was looking for hints of U.S. support for the status quo, he didn’t get one.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.