Mr Saleh, in power since 1978, has already pledged to step down in 2013, but has previously promised to quit.

Yemen's protests have gathered momentum with the success of Egypt's revolution.

"A Yemeni revolution after the Egyptian revolution," chanted demonstrators as they tried to march to the presidential palace in Sanaa on Sunday.

Witnesses said several people were hurt as police armed with batons clashed with stone-throwing protesters. At least 10 people were arrested, said reports.

A rights group has accused the government of colluding with thugs - armed with sticks, clubs, axes and daggers - to suppress the protests.

"The Yemeni authorities have a duty to permit and protect peaceful demonstrations," said Sarah Leah Whitson, of Human Rights Watch (HRW).

"Instead, the security forces and armed thugs appear to be working together."

One of the protesters, Muhamad, told HRW he had been stabbed, beaten and shocked with a Taser gun, and that other people had suffered similar treatment.

"I want the regime to treat us like humans," he said. "So it's my right to express my opinion and express what I suffer from this current regime."

On Saturday, supporters of the president routed demonstrators in the capital.

A day earlier, protesters were dispersed by security forces from Sanaa as they celebrated the toppling of President Hosni Mubarak.

The unrest comes as Mr Saleh is preparing to hold talks with opposition groups on possible political reforms, in an attempt to prevent his overthrow in the manner of Egypt and Tunisia.

He has promised he will stand down and 2013 and that his son will not replace him in office.

Mr Saleh has postponed a visit to the US because of the unrest, state media reports.

An official in Mr Saleh's office said the two countries would "communicate via diplomatic channels" to arrange a new date, according to the Saba news agency.

Yemen, the poorest country in the Arab world, is a key ally for the US in its efforts to combat al-Qaeda in the region.

The country also faces a separatist movement in the south and an uprising of Shia rebels in the north.