Troops loyal to Ali Abdullah Saleh, the Yemeni president, have opened fire on protesters in the capital, Sanaa, killing at least 26 people and injuring hundreds.
Tens of thousands of protestors calling for an end to president Saleh's 33-year rule took to the streets of the capital a day after protesters stormed Yemen's main university.
Mohammad al Qadhi, a Yemeni journalist, said government snipers had fired on demonstrators from rooftops.
"I talked to one of the protesters. He told me shots were fired on chests, legs, and other parts of the body," he said.
Witnesses said security forces and armed civilians opened fire on protesters who left Change Square, where they have camped since February demanding regime change, and marched towards the city centre.
They also used water cannons and fired tear gas, they added.
Freelance journalist Tom Finn said he counted at least 16 bodies piled up in a mosque and most of them were shot in the head.
"Most of them are under 22. I saw one that was 16 years old," he said.
"There are three hospitals in Sanaa filled to the brim with the injured. One doctor said he expects the death toll to rise over to 50 by tomorrow morning."
Munir al Mawri, a Yemeni analyst and a member of the Yemeni Opposition Council, told Al Jazeera that he feared war crimes might be committed in the next few hours as Sanaa is in complete darkness and all telecommunication and internet services have been shut down.
"The regime will try to take advantage of the situation as the whole world remains distracted by issues in Libya and Syria," he said.
Al Mawri said the US was making a big mistake by taking a neutral stance and dealing with the crackdown as if it were a political crisis that can be solved by the opposition and the government.
Earlier on Sunday, government trooops fired mortars into Al-Hasaba district in Sanaa, home to an opposition tribal chief.
Sheik Sadeq al-Ahmar said his fighters did not return fire after they were shelled by the Republican Guard.
He did not want to give Saleh any excuse not to sign a deal to transfer power.
The crackdown on protesters comes as Abdrabuh Mansur Hadi, Yemen's vice-president, will sign a Gulf Arab initiative to arrange for a transfer of power in Yemen "within a week", a high-level Saudi official told reporters.
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"Within a week, the vice-president will sign the Gulf Initiative in the name of the president," said the official, who requested anonymity.
Last week, Saleh authorised Hadi to negotiate a power transfer with the opposition.
The initiative was proposed by the six-nation Gulf Co-operation Council and sets the path for a peaceful transition of power from Saleh, who has ruled Yemen since 1978.
According to the Saudi official, "among the guarantees demanded by Salah are that his son be kept in the next government".
Saleh left the country three months ago for Saudi Arabia where he has been recovering from a June 3 attack on his presidential compound.
The president has since January faced protests over nepotism and corruption from reform activists inspired by the Arab Spring.
On Saturday, thousands of protesters stormed the main university in Sanaa, preventing the first day of school and calling for an end Saleh's rule.
At least six students were injured when thousands of anti-government protesters stormed Yemen's main university.
"No studying, no teaching until the president goes," the students chanted as they marched into the Sanaa University campus, which is has been the centre of Yemen's opposition movement.
The protesters shut the doors of administrative buildings and tore down pictures of Saleh in the dean's offices.
Around the capital, at least 20 other schools were kept closed to students on Saturday because many of the buildings are being used as outposts by government-linked gunmen and soldiers who defected to the opposition, said Fatma Mutahar, principal of Ayesha School in Sanaa and an official with the education ministry.
"Schools are for learning, not to serve as barracks," said Mutahar, who tried to negotiate with the gunmen to leave her school but failed.
More than 60 schools in the southern city of Aden are being used as shelters for people displaced by fighting between government troops and Islamic groups which have taken over several towns during Yemen's turmoil.