Police tried to disperse the demonstrators using tear gas, batons and stun guns, but about 3,000 protesters defiantly continued their march from Sanaa University toward the city center, chanting slogans against President Ali Abdullah Saleh, including "Down with the president's thugs!"
The procession gained momentum with hundreds of students and rights activists joining along the way.
The unrest comes as ties between the U.S. and Saleh have been growing recently over rising alarm in Washington about the activities of al-Qaida in the Arabian Peninsula. The U.S. military has embarked on a plan to deepen its involvement in training Yemen's counterterrorism force to counteract a local affiliate of al-Qaida that has mounted several attacks against the U.S.
Saleh, who has been in office for more than 30 years, has tried to blunt recent unrest by promising not to seek re-election when his term ends in 2013.
He has been contacting powerful tribal leaders in a bid to enlist their support as he attempts to defuse the protests, according to officials familiar with the president's moves. They said that since Saturday, he had personally visited leaders of the powerful Hashed tribe in a number of regions around the capital to solicit their support.
The officials said Saleh feared that his rule would not withstand the pressure of a tribal decision to join the protesters in seeking his ouster, especially since many of them are heavily armed.
For now, said the officials, Saleh was counting on the security forces and armed backers who support his rule in dealing with the protesters.
They spoke on condition of anonymity because of the sensitivity of the subject.
On Tuesday, riot police blocked the main road leading to the city center and clashed with protesters throwing stones. Three protesters were injured and taken to the hospital in ambulances. About 2,000 government supporters staging a counterdemonstration joined the police in battling the protesters.
"We will not back off, whatever the government thugs do," said Tawakul Karman, a senior member of the opposition Islamic fundamentalist Islah Party, She was briefly arrested last month for leading anti-government protests.
"We will retain the dignity of the people and their rights by bringing down the regime," she added.
Rights activist Fathi Abu al-Nassr called the demonstrations "the people's uprising."
"We will not be intimidated by the thugs' attacks," he said, adding that the government funded the demonstrations by supporters, some of whom included senior party members.
Independent lawmaker Ahmed Hashid appealed to international human rights groups to intervene and end the government's harsh treatment of peaceful demonstrators.
By midafternoon, the protesters had dispersed.
In Yemen's southern province of Taiz, more than 5,000 protesters demonstrated for a second day in a main street downtown, and they engaged in an exchange of stone-throwing with police and government supporters. A large number of the protesters had spent the night in the streets, with many others joining them early Tuesday.
Police arrested 120 protesters Monday, but later released 75.
Impoverished Yemen is one of several countries in the Middle East feeling the aftershocks of pro-reform uprisings in Egypt and Tunisia. The protests in Yemen have mushroomed since Friday's ouster of Egyptian President Hosni Mubarak after an 18-day revolt fueled by grievances similar to those in Yemen - poverty, unemployment and corruption.