Security forces backed by army units opened fire Sunday on protesters demanding the ouster of Yemen's longtime president, killing three, an opposition activist said.

In all, tens of thousands of protesters mobilized in several cities and towns, according to activists — the latest installment of daily protests against President Ali Abdullah Saleh that have been staged for almost three months.

One protester was killed in the western port of Hodeida, and two were killed in the southern city of Taiz when elite Republican Guard forces tried to disperse protesters by firing in the air, according to activist Nouh al-Wafi.

In all, more than 140 people reportedly have been killed in the government crackdown on the protesters, who have nonetheless grown in number week after week.

Bushra al-Maktari, an activist in Taiz, said youthful protesters had taken over a main street in that city, setting up tents and expanding a sit-in which started on Feb. 11.

In the southern city of Aden, thousands marched against Saleh and demanded the release of detained protesters.

A deal for Saleh to step down — negotiated by the Gulf Cooperation Council — appeared close to collapse after the president said over the weekend that he would have aides sign it rather than doing so himself. The resulting deadlock threatens to plunge the already unstable nation deeper into disorder.

The mediation plan appeared close to success a week ago, with both opposition parties and Saleh agreeing to it, until the president balked just days before the signing ceremony. The plan called for Saleh to step down within 30 days, with a promise of immunity from prosecution, and for a national unity government to run the country until elections are held.

The violent crackdown on protesters has prompted several top military commanders, ruling party members and diplomats to defect to the opposition, leaving Saleh with dwindling options.

Yemen — the souhtern neighbor of Saudi Arabia — has been plagued in recent years by widespread corruption, a weak central government, a Shiite rebellion in the north, a secessionist movement in the south and one of the most active branches of al-Qaida operating in the remote hinterlands.