Yemen's outgoing president Ali Abdullah Saleh said on Sunday he would leave for medical treatment in the United States and return to continue leading the ruling party, but gave no indication of when he would leave the troubled country.

A source close to Saleh said he was set to leave within the next few hours for the United States via neighbouring Oman.

God willing, I will leave for treatment in the United States and I will return to Sanaa as head of the General People's Congress party, Saleh was quoted by the state news agency Saba as telling a meeting with party officials.

Saleh, who was granted immunity from prosecution under a law passed by parliament on Saturday, also asked Yemenis to forgive him for mistakes made during his rule.

I ask for pardon from all Yemeni men and women for any shortcoming that occurred during my 33-year rule and I ask forgiveness and offer my apologies to all Yemeni men and women, he said. Now we must concentrate on our martyrs and injured.

Thousands of Yemenis protested on Sunday against Saleh's immunity and demanded he be put on trial for offences they say he committed during his rule.

Saleh was granted immunity as part of measures to persuade him to resign after a year of protests against him which have ground the impoverished Arabian Peninsula nation to a halt.

Demonstrators accuse security forces controlled by Saleh and his aides of killing hundreds. Many fear some of his followers will cling to power even once Saleh is gone.

At the capital's airport, dozens of members of Yemen's airforce held a sit-in on the runway to demand the resignation of their commander, Saleh's half-brother, accusing him of corruption.

(Writing by Andrew Hammond; Editing by David Stamp)