BEIRUT, Lebanon — The first explosion tore into a busy street in Damascus. The second, which occurred minutes later as neighbors rushed to help those wounded in the first, may put an end, analysts said, to the effort by Palestinians in Syria to stay out of the country’s widening conflict.

At least 20 people were killed in the shelling on Thursday night, according to the United Nations. Witnesses recalled and an online video portrayed a horrific scene, with gnarled bodies in burning storefronts and women screaming in the streets.

But with responsibility for the attack a matter of intense dispute — the government blames rebels; the rebels blame the Syrian Army — its impact may reach beyond the carnage. Analysts said it could push Palestinian allegiances, already drifting from President Bashar al-Assad of Syria, fully into the rebel camp, greatly assisting the opposition’s struggle for recognition.

“The Palestinian cause is a central cause; it’s a builder of legitimacy and a basis for everything else,” said Joshua Landis, director of the Center for Middle East Studies at the University of Oklahoma. “The regime is clearly very protective of the issue, and the rebels are trying to establish a connection to it as well.”