UPDATED: 4:00 p.m. PDT: The large crowds created a transit nightmare for the D.C. area as they departed the National Mall. Widespread reports of packed facilities, though not on the level of President Obama's inauguration.

UPDATED: 1:40 p.m. PDT: The New York Times' Brian Stelter reports that the National Park Service put attendance at "well over 200,000."  He reports also that Comedy Central streamed the event more than four million times.

A crowd some say is bigger than any they've seen on the National Mall since President Obama's inauguration is eagerly awaiting the start of the Rally to Restore Sanity and/or Fear. 

Headlining the rally, which organizers say isn't about politics, are Jon Stewart and Steven Colbert. Stewart and Colbert will take the stage, which is set up so that it frames the Capitol building, at around 1 p.m. Eastern. 

Many in the crowd are holding signs to support the comedians' message. One sign, according to NYT reporter Brian Stelter, reads "Can See Reason From My House."

Some Democrats hope the rally will reinvigorate young voters, but many at the Mall today have noted that the average age of attendees is higher than what might be expected.

There is no official crowd estimate (despite Stewart's facetious attempts to count), but more than 20,000 people have checked-in to the rally on the social networking site Foursquare.

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To follow the rally:

The Huffington Post, which sent 10,000 to D.C. on 200 buses, has a live blog and live video available here.

The Washington Post is live-streaming the rally here.