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 of pro-reform uprisings in Egypt and Tunisia.
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.
The Ministry of Interior called on people not to heed "suspicious calls for chaos" and to avoid rallies which "obstruct the course of daily life."
Yemen's president Ali Abdullah Saleh also postponed a trip to Washington scheduled for next month due to the "circumstances in the country," the state news agency reported.
Much is at stake in Yemen - a deeply troubled nation strategically located at the mouth of the Red Sea and next door to the world's largest oil reserves. Saleh's weak government is already under pressure from a southern separatist movement and disaffected tribesmen around the country.
The U.S., however, is most worried about an al-Qaida offshoot that has taken root in Yemen's mountains in the last few years and used it as a haven 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.
Opposition parties set several conditions for joining talks with the government, including a definitive timetable for "constitutional, legal and economic reforms."
The parties also demanded that Saleh remove his sons and other relatives from army, security and government posts.
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 on to his son.
Several people were injured in Sunday's demonstrations, and police detained 23 protesters, witnesses said.
The Interior Ministry, which oversees the internal security forces, accused the protesters of "spreading sabotage and chaos" and "threatening security and stability."
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. Much of the population suffers from malnutrition as well.
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.