Yemeni police opened fire on protesters in the capital Sanaa on Tuesday, wounding at least 98 people demonstrating for an end to President Ali Abdullah Saleh's 32-year rule, witnesses said.
Three of the wounded were in serious condition, they said.
Police and security agents in civilian clothes fired rubber bullets and tear gas as they tried to prevent people from joining thousands of protesters who have camped out for weeks in front of Sanaa University, the witnesses told Reuters.
There was no immediate government comment.
Police brought out water cannon and placed concrete blocks around Sanaa University, the rallying point for anti-Saleh protest that had been quiet in recent days, after weeks of fierce clashes across the country between government loyalists and protesters that killed at least 27 people.
Witnesses reported seeing armored vehicles and personnel carriers headed to the area of the university.
"It's a massacre," said opposition spokesman Muhammad Qahtan. "It is a crime by security troops against students engaged in a peaceful sit-in."
Yemen has been rocked by weeks of protests against Saleh, inspired by recent uprisings in Egypt and Tunisia that drove out those nations' leaders. Saleh, a key U.S. ally in the campaign against al-Qaida, has been in power 32 years. In a sign that the protests are gaining traction, graffiti calling for Saleh to step down surfaced Tuesday in his birthplace, village of Sanhan, for the first time since the protests began.
Also Tuesday, thousands of inmates rioted at the central prison in Sanaa, taking a dozen guards hostage and calling for Saleh to step down. At least one prisoner was killed and 80 people were wounded, police said.
The unrest started late Monday when prisoners set their blankets and mattresses ablaze and occupied the facility's main courtyard, a security official said.
Prison guards fired tear gas and gunshots in the air but failed to subdue the rioters, the security official said, speaking on condition of anonymity because he was not authorized to talk to the media. The prison revolt was still going on Tuesday afternoon, he said.
Residents close to the prison reported hearing gunfire and blasts and said troops were bringing in military reinforcements.
Abdelrahman Burman, a lawyer who heads a human rights organization called "Sajin" — Arabic for prisoner — said at least one inmate was killed by a bullet fired by riot police and more than 80 people were wounded, including 20 policemen and prison guards.
He said about half of the wounded were shot and the rest suffered breathing problems and fainting because of the tear gas.
A police official confirmed the casualty numbers. He spoke on condition of anonymity because of the sensitivity of the situation.
Also Tuesday, large demonstrations were held in various parts of the country. In the southern port city of Aden, a crowd of women joined a demonstration after a young protester was shot in the head and critically wounded during a rally there the previous day.
Tens of thousands took to the streets in the Ibb province, calling on the government to bring to justice those responsible for a deadly attack there Sunday. Opposition activists blamed "government thugs" who descended on protesters camped out on a main square. One person was killed in that violence and 53 people were hurt.
Even before Yemen was hit by the wave of protests, the country was growing increasingly chaotic with a resurgent al-Qaida, a separatist movement in the south and an off-on Shiite rebellion in the north.
In an attempt to quell escalating protests, the president called for national dialogue after meetings Monday with the country's top political and security chiefs. The state-run news agency said the conference would be held Thursday and would include thousands of representatives from across Yemen's political spectrum.
But opposition leader Yassin Said Numan said there would be no dialogue unless Saleh agreed to step down by year's end.
Saleh's recent pledge not to run for re-election in 2013 has failed to quell the protests.
In other Middle East unrest:
Christian dead in clashes with Muslims: Thousands of Christians and Muslims clashed Tuesday, with one Christian man killed and scores wounded as anger rose over the burning of a church in a Cairo suburb. It was the second burst of sectarian fighting in as many days and the latest in a string of violent protests over a variety of topics as simmering unrest continues nearly a month after mass protests led to the ouster of President Hosni Mubarak. The violence erupted after Coptic Christians held protests in several locations in Cairo against perceived persecution by the country's Muslim majority. Egypt's military rulers have pledged to rebuild the church. But the protesters said they wanted more steps to improve the status of Christians. About 2,000 of them cut off a main road running on the eastern side of the city and pelted motorists with rocks. Another crowd of about 1,000 protested outside the TV building in downtown Cairo. The group which included a group of garbage collectors, who are predominantly Christian, demanded equal rights and better quality of life. The clashes broke out when they were confronted by Muslims, witnesses said.
Elsewhere in Cairo, a protest by hundreds of Egyptian women demanding equal rights and an end to sexual harassment turned violent Tuesday when crowds of men heckled and shoved the demonstrators, telling them to go home where they belong.
Even before Egypt's uprising unleashed a torrent of discontent, tensions had been growing between Christians and Muslims in the country. Also, an Egyptian court rejects an appeal by ousted President Hosni Mubarak and his family against a top prosecutor's move to seize funds that could total in the billions of dollars. The decision clears the way for a criminal investigation and a possible trial of Mubarak, who stepped down Feb. 11 after 18 days of massive protests.
Protest in Kuwait: More than 1,000 protesters call for sweeping political changes in Kuwait as the surge for reforms around the Arab world moves into another Gulf state. Security forces stand by as demonstrators move into an area outside a building holding key offices including those of Kuwait's emir and the prime minister, who is accused by pro-reform groups of stifling political freedoms and muzzling dissent.
Rally in Oman: More than 150 protesters rally outside Oman's state television headquarters in the capital Muscat to call for greater press freedoms. The peaceful demonstration comes a day after Sultan Qaboos bin Said makes another concession to protesters' calls for more jobs and political openness by dissolving the office overseeing economic affairs.
Syria frees activist: Syria releases a leading lawyer and human rights activist who was imprisoned since 2009 for "spreading false information" after giving a television interview that criticized excessive government security and corruption. Haitham al-Maleh, who is 80 and has diabetes and thyroid problems, was convicted in July and sentenced to three years in prison.