Here's what we seem to know at around 10:45 eastern time this morning:
Al Jazeera (AJE) is reporting three people dead, based on their correspondent's eyewitness of bodies. AP is reporting "at least one" dead. AFP is reporting that one woman was hit by police on the head with a baton but that "it's unclear whether anyone else was injured."
AJE and AFP are reporting that members of the CDC opposition party fired into the crowd, prompting police response. AP is sticking with the it's-all-rather-unclear-at-this-point approach.
Photojournalist Glenna Gordon reported by email that the violence began about 1:30 p.m. Monrovia time. She said that phone lines were cut, and in a follow up email that they are still out. She also said that Liberian National Police fired tear gas; and CDC members chanted, "Tonight, tonight there will be a massacre."
By 11 a.m. eastern time, Gordon said by email that the crowd had grown to 300 outside the headquarters, with "lots of verbal confrontation... CDC guys do not seem to be armed, just loud." UN peacekeepers are present at the scene, with tanks. Gordon reports a confrontation between a UN peacekeeper and a Liberian National Police officer. "The LNP officer pulled his gun on peacekeepers. He didn't fire."
Gordon said by email that the violence happened at CDC headquarters on Tubman Boulevard in Congotown, where about 100 people had gathered inside and another 150 outside, by her quick estimate.
Twitter conversation so far is being driven by wire reporting, little on the ground reporting. This may be related to the phones being out, but may have more to do with how Twitter is and isn't used on the ground and by different constituencies in Liberia.
Opposition politician William Tubman, until recently a CDC candidate for president in tomorrow's run-off election, organized a boycott of the election in protest of what he alleges is corruption in favor of incumbent Ellen Johnsnon Sirleaf. The violence appears to have begun out of the boycott, though the catalyst is unclear, says AP.
I'll update with more from Gordon as I get it and with more information and/or corrections from wire news sources as the story progresses.
UPDATE: Gordon's photos of the riot, and a brief description of what she witnessed, here. (link corrected; apologies for error and h/t to alert reader, aka my mom, for pointing out the bad link!)