Yemen's opposition held mass protests on Monday, escalating demands for the immediate departure of President Ali Abdullah Saleh after the ailing leader said his future should be determined at the ballot box.

Tens of thousands of protesters marched in two demonstrations, one for men and another for women, from Change Square, epicentre of anti-regime protests which have rocked Yemen since late January, an AFP correspondent reported.

In Taez, Yemen's second largest city, hundreds of thousands marched from Jamal street to the protest encampment at Freedom Square.

Mohammed Qahtan, spokesman for the parliamentary Common Forum of opposition parties, said Saleh clearly had no intention of stepping aside.

"Saleh has shown in his address that he is still clinging to power, and that he refuses the Gulf initiative that provides for a political transfer," Qahtan said.

"After the speech of the president, there is no way to reach a political solution, and the revolution will intensify," he added.

Saleh, who unexpectedly returned Friday to Yemen after a months-long stay in Saudi Arabia for treatment from bomb blast wounds, late on Sunday challenged the opposition to head to early elections.

"You who are running after power, let us head together toward the ballot boxes. We are against coups," Saleh said in a speech aired on state television on the 49th anniversary of the September 26, 1962 revolution that saw Yemen proclaimed a republic.

"We have repeatedly called for power transfer through the ballot box... let us head together to dialogue and peaceful rotation over power through the ballot box and early presidential elections as the Gulf initiative stipulates," he said.

The 69-year-old president has repeatedly refused to sign a power transfer deal brokered by the Gulf Cooperation Council (GCC) in terms of which he would hand power to Vice President Abdrabuh Mansur Hadi in return for immunity from prosecution.

But he said on Sunday he had authorised Hadi to sign the deal on his behalf.

"We are committed to implementing the Gulf initiative as it is, and to signing it by Vice President Abdrabuh Mansur Hadi, whom we have authorised in a presidential decree," he said.

At Change Square, youth protesters were prompt in rejecting Saleh's speech. "The youth will not accept," said Walid al-Amari, a leading member of the youth protest committee, addressing demonstrators at the square during the night.

"They will not give up until they achieve all the goals of the revolution," he added, referring to demands that the veteran leader quit power immediately.

Saleh's speech came hours after his security forces opened fire when tens of thousands of people marched in Sanaa demanding he be put on trial for crimes committed during his decades-long rule.

Eighteen people were wounded, including one who is now in a coma, according to medics.

The Gulf-sponsored deal was meant to be finalised last week but efforts by international and regional mediators were torpedoed by intense fighting between Yemeni security forces backed by Saleh loyalists on the one side and by defected army units and dissident tribesmen on the other.

The violence in the capital, which according to figures obtained from medics, the opposition and tribal sources left 173 people dead in one week, calmed at the weekend though the capital remains tense.

In other unrest, a general from the elite Republican Guard was killed and 30 troops were captured when tribesmen opposed to Saleh overnight attacked their base north of Sanaa, the defence ministry and tribal sources said.

Four of the attackers were killed in the attack in the town of Nihm, about 60 kilometres (42 miles) north of Sanaa. Saleh has come under pressure from the GCC, the United Nations and the United States to relinquish power.

On Sunday, King Abdullah of Saudi Arabia, Yemen's powerful and wealthy Gulf neighbour, urged Yemenis to implement the Gulf Initiative.

"We see that the Gulf Initiative is still the exit to resolve the Yemeni crisis and prevent the situation (there) from getting worse," he said .

On Saturday, GCC ministers condemned the violence in Yemen and echoed US and UN calls urging Saleh to 'immediately' sign the initiative.