President Ali Abdullah Saleh of Yemen agreed to step down on Wednesday after 33 years in power, becoming the fourth Arab leader swept away by protests this year and launching his violence-wracked country into a new era of uncertainty. 

The agreement transfers power to vice president Abed Rabbo Mansour Hadi within 30 days, kicking off a process that will elect a new president within three months and hand significant power to opposition parties, according to Yemen state television and opposition officials.

Protesters massed in San'a's central Change Square following the announcement, angrily denouncing the agreement's provision of immunity from prosecution for Mr. Saleh. Protesters demanded that Mr. Saleh be trailed and his immunity be stripped, and blamed the country's established opposition parties for letting the leader escape after months of bloodshed they say killed more than 1,000 people.

"No immunity to the killer Saleh," they shouted. "We will not go home until justice prevails."