Sana'a: At least 20 protesters were killed in the Yemeni capital Sana'a on Sunday when security forces opened fire on one of the biggest demonstrations against President Ali Abdullah Saleh in recent months, a Reuters witness said.

Scores were wounded when protesters tried to break through lines of police who also sprayed them with tear gas as tens of thousands spilled out of Change Square, where many youths have camped out since early this year demanding an end to Saleh's 33-year rule.

Injured protesters were rushed into a field hospital in the square on stretchers. "This is the worst day I've seen in three months. We're expecting more dead to come in," said doctor Jamal Al Hamdani, who was treating patients.

A Reuters witness saw dozens of men slumped on the ground, overcome by tear gas inhalation. Men on motorbikes and ambulances whisked them away from the scene.

Saleh, recovering in neighbouring Saudi Arabia from a June assassination attempt, is holding onto power despite international pressure to quit and eight months of protests which have paralysed the country.

The Defence Ministry said on its website protesters threw petrol bombs, setting a police car ablaze. State media blamed gunmen belonging to opposition parties for opening fire on the march. Frustrated by Saleh's tenacity and their failure to dislodge him, protesters are seeking to ratchet up demonstrations, which have dragged into their eighth month.

"Escalation, escalation," they chanted, flooding side streets where there were large numbers of security forces and armed anti-Saleh tribesmen, who have thrown their weight behind the popular uprising.

Earlier, fighting broke out in a northern district of Sana'a, the latest breach of a ceasefire between tribesmen opposed to Saleh and troops loyal to him.