Saleh's opponents have been holding near-daily protests in major towns and cities demanding his exit [AFP]

At least 12 people have been killed and 190 others wounded after Yemeni security forces opened fire on a massive anti-government demonstration in the capital Sanaa, a doctor at the scene said.

"Many of the dead and wounded were shot in the head and torso," Mohammed al-Ibahi told The Associated Press on Wednesday.

The Sanaa protesters were trying to reach an area beyond the district where they have been camped out since February, and were demanding Ali Abdullah Saleh, the president, leave office immediately, witnesses said.

Residents in at least 18 cities and towns across the country launched a civil disobedience campaign on Wednesday protesting against a plan which would give Saleh a month-long window to resign.

The opposition bloc, which has warned government-orchestrated violence against protesters could derail a draft deal to end the political standoff, agreed on Tuesday to take part in a transitional government under the Gulf-negotiated pact, which gave Saleh immunity and his family.

The balance of power has tipped against Saleh, who has ruled for 32 years, after weeks of violence, military defections and political reversals.

Seeing political allies desert him en masse, the Yemeni leader agreed in principle to a proposal by Gulf Co-operation Council (GCC) foreign ministers to step down in exchange for immunity from prosecution for himself and top aides.

But analysts say that allowing Saleh, who has been a key ally of the West against al-Qaeda, to stay on for another month could leave a room for further trouble in the poorest Arab state long on the brink of collapse.

The risk of Yemen descending into chaos is a major worry for Saudi Arabia and the United States, which fear an active al-Qaeda wing could strengthen a foothold in the Arabian Peninsula.

Wednesday's campaign is the latest in Yemen's uprising that started in early February, inspired by revolts across the Arab world.

More than 130 people have been killed so far by security forces and Saleh's supporters at massive near-daily protests.