"If they want me to quit, I will only leave through the ballot box," he told a news conference as vast crowds of protesters, among them opposition MPs, gathered outside Sanaa University to demand he step down.
"The opposition are raising the level of their demands, some of which are illicit," Saleh said, as tens of thousands of protesters took to the streets in centres across the country, including Sanaa, the southern city of Aden, the northern city of Saada -- stronghold of Shiite Huthi rebels -- the western port city of Al-Hudaydah and in Taez, south of Sanaa.
While most of the protests passed off peacefully, in Aden, where clashes between protesters and police have occurred daily since February 16, medics and witnesses said police opened fire on demonstrators, killing one and wounding four.
According to an AFP tally based on reports by medics, 12 people have been killed and dozens more wounded since February 16 when protests first erupted in Yemen against Saleh, who has been in power since 1978.
The interior ministry on Monday put the death toll at four, according to the official Saba news agency.
A tribal leader in the country's north told AFP tens of thousands of people also demonstrated in the group's stronghold of Saada to demand the president step down.
The Zaidi Shiite rebel movement from 2004 fought six wars with Saleh's government before signing a truce in February 2010.
Around a dozen opposition MPs, who vowed to take to the streets in a statement issued on Sunday, have also joined students who have been protesting for the past nine days.
Security forces surrounded the protesters Monday as they gathered in a square near Sanaa University, which they have dubbed Al-Huriya (Liberty), brandishing banners declaring: "People want change," "The people want to overthrow the regime" and "Leave!"
The protesters, who have set up tents at the square, vowed to stand firm despite Saleh having announced the formation of three committees to examine security, medical care and nutrition in Yemen, one of the most impoverished countries in the Arab world.
"The students will not leave unless either the president falls or they fall dead," said one of the students, Muamar al-Haidari.
Opposition MP Ahmed Saif Hashed told AFP Saleh was bound to follow in the footsteps of Tunisia's strongman Zine El Abidine Ben Ali and Egyptian Hosni Mubarak, both toppled in popular uprisings this year.
"Now we are waiting for (Libya's Moamer) Kadhafi, and the Yemeni president will follow," Hashed said.
Loyalists demonstrating in support of Saleh in central Sanaa have almost daily broken up students protests using batons and stones, with police also using violence that has left scores of demonstrators injured.
In the western port city of Al-Hudaydah, three protesters were wounded in clashes with the regime's loyalists on Monday, demonstrators told AFP.
In Huta, the capital of the southern Lahij province, three people were wounded when security forces fired tear gas and live rounds at demonstrators who marched toward the security headquarters to call for the release of prisoners, witnesses and medics said.
Demonstrators then looted and burned down a local government building, witnesses said.
And in Taez, south of Sanaa, protests continued for the 11th straight day as tens of thousands gathered in a square near the municipality building, some setting up tents.
Saleh has outlived the Cold War division, civil war and an Al-Qaeda insurgency but is now scrambling to see his term through to the end as sustained popular uprisings in Sanaa and Aden test his grip on power.
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