Gunfire broke out in the Yemeni capital and at least four people were killed as security forces attacked protesters gathered for the largest of eight days of anti-government demonstrations.
Police swinging batons and government supporters hurling rocks were seen attacking demonstrators near Sanaa University. One man was killed after a hand grenade was thrown into a crowd of anti-government protesters in the city of Taiz near the Red Sea, al-Jazeera reported.
More than 20,000 people streamed out of Friday prayers and began to march on the presidential palace in what was billed a "Friday of Fury", demanding that President Ali Abdullah Saleh step down after 32 years in power.
"After Mubarak, Ali," they chanted. Eighteen days of anti-government protests in Egypt forced President Hosni Mubarak to resign a week ago, ending his 30-year rule.
The crowds in Yemen were the largest so far, overwhelming police who were trying to contain them within the vicinity of the university by blocking roads and forming human walls.
Protests were also reported in Aden, Yemen's Red Sea port city, where people set fire to a council building and a police station.
The Interior Ministry denied a statement that undercover security officials were leading pro-government rallies and attacking protesters.
The US Embassy in Sanaa called the rise in violence "disturbing", saying it contradicted promises Mr Saleh made to allow peaceful demonstrations.
"We have seen reports that government of Yemen officials were present during these attacks," the embassy said in an emailed statement. "The attacks are contrary to the commitments that President Saleh made to protect the right of Yemeni citizens to gather peacefully to express their views."
Mr Saleh said on February 2 that he would not seek to extend his term when it expires in 2013 and that his son wouldn't succeed him as president.