The protests have mushroomed since crowds gathered Friday to celebrate the ouster of Egyptian President Hosni Mubarak after an 18-day revolt fueled by similar grievances. Yemen is one of several countries in the Middle East feeling the aftershocks, as pro-reform demonstrators take inspiration from the uprisings in Egypt and Tunisia.
Much is at stake in Yemen - already a deeply troubled nation - if the pressure on President Ali Abdullah Saleh further erodes stability.
The U.S. is most worried about an al-Qaida offshoot that has taken root in Yemen's mountains in the last few years and used the haven as a base to plot attacks beyond the country's borders, including the failed attempt to blow up a U.S.-bound airliner in December 2009 by an attacker with a bomb sewn into his underwear.
Saleh - in power for three decades - is quietly cooperating with the U.S. in efforts to battle the al-Qaida franchise, but his government exercises limited control in the tribal areas beyond the capital. The U.S. is funneling him military aid and training.
The country's security forces, however, are already stretched thin on two other fronts: Since 2004, they have struggled to contain a serious rebellion in the north by members of the Zaidi sect of Shia Islam who complain of neglect and discrimination. At the same time, police and army forces are clashing with a secessionist movement in southern Yemen, which was a separate country until 1990.
Now, the protests calling for the president's ouster over corruption allegations and other complaints are adding another serious challenge to the list.
Saleh has tried to defuse the unrest by promising not to run again when his term ends in 2013 and guaranteeing that he will not seek to pass power to his son.
On Sunday, uniformed police used truncheons to stop protesters, many of them university students, from reaching the capital's central Hada Square. Witnesses said plainclothes policemen wielding daggers and sticks also joined security forces in driving the protesters back.
Several people were injured and police detained 23 protesters, witnesses said.
The crowds took up the protest cry that became famous in Tunisia and then in Egypt, shouting, "The people want to overthrow the regime."
They have also tried to reach a square in the capital with the same name as the plaza that became the epicenter of Egypt's protest movement: Tahrir, or Liberation, Square.
Seeking to stop them, the police have ringed the square with barbed wire and bussed in government supporters to set up a tent camp and occupy and defend the square around the clock.
On Sunday, local officials gave police and government supporters free portions of the leafy narcotic qat, which many Yemenis chew throughout the day, witnesses said.
Yemen is the Arab world's most impoverished nation. Its main source of income - oil - could run dry in a decade, and the country is also rapidly running out of water.
Yemen has been the site of anti-U.S. attacks dating back to the 2000 bombing of the USS Cole in Aden harbor, which killed 17 American sailors. Radical U.S.-born cleric Anwar al-Awlaki, thought to be hiding in Yemen, is suspected of having inspired some attacks, including the deadly 2009 shooting rampage at Fort Hood, Texas.
