Update 5 p.m. ET: NBC News' Richard Engel reports on Twitter:
But supporters of President Hosni Mubarak have managed to take the high ground on October Bridge. With both sides using metal "shields" to "advance like Roman legions," he says, "it is a running battle."
Protesters worry that pro-Mubarak forces, now in retreat, will bring in reinforcements.
Update 3:30 p.m. ET: NBC News' Brian Williams reports from Cairo that the situation in Tahrir Square is "clearly deteriorating." He says he just heard a volley of weapons fire.
Update 3:23 p.m. ET: NBC News' Yuka Tachibana and Paul Nassar report from Cairo that they can hear gunfire from the back of the building they are in.
NBC News' Richard Engel reports from Cairo that pro-Mubarak supporters have taken positions in all four corners, trapping protesters in the middle:
NBC's Richard Engel reports the latest on the clashes in Egypt from Cairo.