The "March for Life" rally from Taiz to the capital Sanaa began on Tuesday [AFP]

Yemeni security forces have killed 13 protesters in an attack on a crowd of more than 100,000 protesters peacefully marching into the capital to protest against President Ali Abdullah Saleh.

"Thirteen people were killed and 50 others were wounded by live rounds," a medical official said on Sunday.

The medic from a field hospital in the capital said that 150 other people suffered from breathing difficulties due to tear gas inhalation.

Saleh had earlier said he would go to the United States in order to allow an interim government to prepare for an election to replace him, but did not specify when he would leave.

The protesters had marched for four days and 320km on foot to pressure the government not to give Saleh immunity from prosecution, in the first march of its kind in the impoverished country.

After protesters arrived at the southern entrances to the capital, forces of the elite Republican Guard fired on them with automatic weapons, tear gas and water cannons, sparking hours of clashes.

Saleh travels

Saleh, speaking to reporters after forces loyal to him fired at protesters demanding he face trial for killing demonstrators over 11 months of protests, said he had no designs on staying in power.

"I will go to the United States. Not for treatment, because I'm fine, but to get away from attention, cameras, and allow the unity government to prepare properly for elections," he said on Saturday.

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"I'll be there for several days, but I'll return because I won't leave my people and comrades who have been steadfast for 11 months," he said. "I'll withdraw from political work and go into the street as part of the opposition."

"This march is the biggest violation of the Gulf initiative," Saleh told reporters in Sanaa a few hours after the opposition accused his troops of attacking the protesters.

Tens of thousands of demonstrators set off from the southern city of Taiz on Tuesday for the march to the capital.

The demonstrators, part of the "March for Life" rally, arrived in Sanaa in mid-afternoon on Saturday but were blocked in a southern suburb, witnesses said. Around 90 others were wounded, doctors told the Reuters news agency.

Activist Waddad al-Dhalie said that a number of female protesters were also injured with bullets and tear gas.