Hundreds of protesters killed in Syria

>>> good evening. as we come on the air tonight, all eyes are on the middle east and two unfolding crises. in syria , the latest attempt to crush the anti-government uprising may have been the deadliest yet with hundreds reportedly killed in a bombardment of syria 's third largest city. tonight, president obama is calling on syria 's president assad to step aside and allow a democratic transition to proceed immediately. but today, a u.n. resolution calling for assad to leave failed when china and russia refused to support it. and also iran is raising the stakes in its tense standoff with israel. we've got our team in place from washington to across the middle east . we'll start in cairo with nbc's ayman mohyeldin who is monitoring the latest events. what's the latest?

>> reporter: good evening, lester. it was an attack that happened under the cover of darkness, but by daylight, the human toll was apparent for the world to see. because it's hard for journalists to report from inside syria , we have been relying on footage from the act is lists to tell this story. so we have to warn our viewers that some of the images are disturbing. activists are describing it as the single deadliest day in the 11-month syrian uprising. a barrage of mortars raining down on the city of homes, killing more than 200 people. amateur footage which we couldn't independently verify captured the horror of the nighttime attack as some are calling a massacre.

>> we're trying to remove the bodies from the middle of the street and we get shot at. we tried to get doctors and medication to the area.

>> reporter: homs has seen the most intense fighting. video on pro government tv stations portrayed life as normal. by daylight, thousands defied the risk of continued violence, taking to the streets and mourning their dead in funerals. not just in homs, but across the country. syrians continue to grieve russia and china block the united nations resolution that was trying to end the violence. the syrian opposition says it's not deterred.

>> the people will continue regardless of what's going on on the internal stage. people will continue democrat demonstrating.

>> reporter: it triggered demonstrations in london, cairo and while they're dismayed by the u.n.'s actions, thousands poured on to the streets and their resolve has not been broken. lester, a senior opposition leader tells us it would be suicide to stop the protests now because the assad government would seek out and punish anyone that dared to speak out and demonstrate against its in the past 11 months.

>> thank you.