Forces loyal to Yemen's Ali Abdullah Saleh fired guns, tear gas and water cannons to keep tens of thousands of protesters away from the president's compound in Sanaa on Saturday, killing at least one woman, witnesses said.

In southern Yemen, gunmen killed a Briton of Yemeni origin and wounded a soldier accompanying him in an attack on an oil company vehicle that a local official blamed on highway robbers.

In Sanaa, residents said shots rang out when riot police and troops blocked activists who had reached the capital chanting No to immunity, at the climax of a mass march that began days earlier in the city of Taiz, 200 km (125 miles) to the south.

The protesters were denouncing a deal granting Saleh immunity from prosecution for his part in a violent crackdown on months of demonstrations against his 33-year rule. In return, the president has handed over his powers to his deputy, pending a presidential election scheduled for February.

One woman marcher was killed, said activists. Medics said 10 people were wounded, some by bullets or tear gas canisters.

The immunity deal was crafted by Yemen's rich neighbours in the Gulf Cooperation Council (GCC) to ease Saleh from power and avert civil war in a country that hosts an ambitious wing of al Qaeda and sits next to vital oil shipping lanes.

An interim government preparing for the presidential election has promised to separate pro-Saleh troops from tribal militia and rebel army units in Sanaa and elsewhere.

If he goes, Saleh would be the fourth leader to surrender power after mass protests that have redrawn the political map in North Africa and the Middle East in the past 12 months.

FOR SHAME

Protesters, many of them young, chanted For shame, the blood of the martyrs has been sold for dollars, referring to the immunity deal, which was endorsed by a coalition of opposition parties that are part of the interim government.

Witnesses said pro-Saleh troops, some in tanks and armoured vehicles, deployed on streets leading to the presidential compound to stop any attempt by protesters to approach it.

They also used tear gas to try to turn back protesters in the Sabaeen district of the capital, the witnesses said.

The marchers later retreated and headed for Change Square, a rallying point for the protests which began in January.

Protesters want a purge of Saleh relatives who still hold key posts in the military and security forces.

The interim government faces multiple challenges including lawlessness and a secessionist movement in the south, where Islamists have seized chunks of Abyan province.

In the southern port city of Aden, a grenade blast, apparently the work of feuding gangs, killed one person and wounded five at a market late on Friday, a local official said.

Separatist sentiment is running high in the south, formerly a socialist republic that fought a civil war with Saleh's north in 1994 after four turbulent years of formal union.

Top oil exporter Saudi Arabia shares U.S. fears that more instability could embolden al Qaeda's branch in Yemen.

(Reporting by Mohammed Ghobari; Additional reporting by Mohammed Mukhashaf in Aden; Writing by Joseph Logan and Firouz Sedarat; Editing by Alistair Lyon)