It was the largest rally on climate change in U.S. history. The rally was sponsored by national groups like the The Sierra Club, 350.org, Hip Hop Caucus, and Food & Water Watch, plus many local groups.
Participants came from 30-plus states.
The group met near the Washington monument to hear talks from people like Bill McKibben, founder of the environmental activist group, 350.org, Sierra Club Executive Director Michael Brune and Sen. Sheldon Whitehouse, D-R.I..

We then walked around the White House, with chants like:
"Hey, Obama, We don't want no climate drama."
"Hey Obama just say no; The keystone pipeline has to go."
"Mr. Obama, yes you can; Stop the dirty pipeline plan."
Other improvised chants were made up along the route around the White House.
Other's sang Woody Guthrie's, "This Land is Your Land".

Opposition to the pipeline led the Sierra Club to lift a long-standing ban on civil disobedience for the sake of opposing the pipeline's construction. The club's executive director, Michael Brune, was arrested at a Wednesday protest outside the White House.

It was my first big protest rally since People's Park in Berkeley in 1971 *.
Some observations.
My estimation of the demographics: 20 somethings - 60%, Over 60 - 25%, 30-60 - 15%. It took 1 1/2 hrs. to walk the 1.7 miles around the White House, in what sometimes felt like a human traffic jam on a 36° windy day.
President Obama was in Florida, playing golf with Tiger Woods.

I can't say that the Keystone XL oil pipeline is on my top 10 list of environmental, political or social problems in the world, but Big Oil's stranglehold on Washington is in my top 2 or 3 and KXL seems to be the best poster child for that right now.

See:
Pictures at New Jersey SierraClub Facebook
Huffington Post
and Keystone XL (KXL) oil pipeline

* I have participated in local rallies and testified at planning board meetings and Board of Public Utilities hearings, and lobbied my congressman on environmental issues, but this is my first "big" rally.