The protesters fought off attacks by police and government supporters swinging batons and daggers. Municipal vehicles ferried sticks and stones to the pro-government side, witnesses said.
For seven straight days, protests have hit the capital, Sanaa, and other cities in the Arab world's poorest country, a mountainous territory wracked by tribal conflicts, armed rebellion and other serious woes.
Inspired by uprisings in Egypt and Tunisia, Yemenis have poured into the streets to demand the ouster of President Ali Abdullah Saleh after 32 years in power three years more than Egypt's Hosni Mubarak. Their main grievances are poverty and government corruption.
Saleh's promises not to run for re-election in 2013 or to set up his son to succeed him have failed to quiet the anti-government storm sweeping Yemen and other nations in the region. Similar protests also swept into Libya and Bahrain this week.
The Yemeni president is an important US ally in fighting al-Qaida. The terror group's Yemen-based offshoot has been linked to attacks beyond Yemen's borders, including the failed attempt in December 2009 to blow up a Detroit-bound airliner.
Today's protests began with small gatherings of students marching toward downtown Sanaa. Many other people joined them as clashes broke out with police and government supporters. The number of protesters reached about 6,000.
"People want to topple the president, people want to topple the regime," they chanted.
Witnesses said police fired shots into the air to disperse the protesters. A dozen protesters and an unknown number of policemen were injured.
Security officials said police arrested about 50 protesters. Clashes were reported today in several other towns, including the port city of Aden.Seeking to hold on to the momentum, activists have called for a "day of rage" tomorrow.
Yemen's official news agency, Saba, reported that President Saleh chaired a meeting of the army's top officers last night to discuss the turmoil. The Defence Council later called for calm.
Another appeal to end the protests came from Yemen's most prominent religious figure, Sheik Abdul-Majid al-Zindani, who is close to Saleh. The United States considers him an al-Qaida-linked terrorist. "Change by street protest is rejected. It leads to chaos," al-Zindani said