Around 2,000 police flooded the streets of the capital, Sanaa, trying to halt protests. Firing in the air, police locked the gates of Sanaa University with chains to prevent thousands of protesting students inside from marching out join crowds demonstrating elsewhere in the city, witnesses said.
A call spread via Facebook and Twitter urging Yemenis to join a series of "One Million People" rallies on a so-called "Friday of Rage" in all Yemeni cities, demanding the ouster of President Ali Abdullah Saleh, in power for 32 years.
"We will remain in the streets until the regime's departure," according to a statement posted on Facebook. Copies signed by a group named the Feb. 24 Movement were distributed among youth via e-mail. The group is taking that name because organizers hope to have their biggest protest on that day next week.
Taking inspiration from the toppling of autocratic leaders in Egypt and Tunisia, the protesters are demanding political reforms and Saleh's resignation, complaining of poverty, unemployment and corruption in the Arab world's most impoverished nation.
Saleh has tried to defuse protesters' anger amid the unprecedented street demonstrations by saying he will not run for another term in 2013 and that he will not seek to set up his son, Ahmed, to succeed him in the conflict-ridden and impoverished nation.
Protesters still chanted slogans against the president's son Wednesday.
Saleh has become a key U.S. partner in battling al-Qaida in the Arabian Peninsula, the terrorist network's offshoot in Yemen. The group's several hundred fighters have battled Saleh's U.S.-backed forces and have been linked to attacks beyond Yemen's borders, including the failed attempt to blow up a Detroit-bound airliner in December 2009. The U.S. military plans a $75 million training program with Yemen's counterterrorism unit to expand its size and capabilities in the nation's mountainous terrain.
It's a difficult balancing act for Saleh, who has been criticized as being too close to the United States.
Yemeni state TV reported that Saleh has been holding meetings since Sunday with heads of tribes to prevent them from joining the anti-government protests. He met Wednesday with the Supreme Defense Council to discuss developments in the country.
Government supporters massed outside Sanaa University during Wednesday's protests, waving pictures of Saleh. Some threw stones at the protesters inside, as police tried to keep them away from the university gates. Four people were hurt in scuffles, witnesses said.
Demonstrations by thousands shouting, "Down with Ali Abdullah Saleh," also took place in Taiz, Yemen's second largest city, and the southern port of Aden.
Riot police in Aden fired live ammunition, rubber bullets and tear gas in fierce clashes with thousands of demonstrators. Two protesters, including a 23-year-old shot in the head, were killed, a security officer said.
Twenty others were wounded, at least one seriously, according to a medical official, who like the security officer spoke on condition of anonymity because they were not authorized to talk to the media.
The protesters, who included students and workers, set tires ablaze in Aden's Mansoura district, witnesses said. Heavy gunfire rattled residents, and many closed their shops and stay home. Protests also erupted in Aden's most populated district, Sheikh Othman, and another section of the city.
As some protests marched toward the city center, Republican Guards armored vehicles blocked entry points to Crater, Aden's ancient historic port district built in the crater of an extinct volcano on a peninsula off the mainland.
In central Taiz, about 270 miles south of Sanaa, protesters have been camping in Safir Square, saying they will not leave until Saleh steps down. Just like in Cairo's Tahrir Square, protesters have organized a makeshift camp in the city center, with medical teams, cleaning crews and security to protect them from outside attacks, said Ghazi al-Samie, a lawyer and activist.
Al-Samie said thousands have joined the protests in recent days in Yemen's second-biggest city.
About 120 judges held a protest in front of the Ministry of Justice in Sanaa, calling for an independent judiciary and better salaries. It was the first demonstration by judges in Yemen.
Saleh's government is weak - its control barely extends beyond the capital and is dependent on fragile alliances with powerful tribes - and it faces other serious challenges.
For more than six years, government forces have been battling a sporadic armed rebellion in the north. A secessionist movement by once-independent southern Yemen also is heating up.
Meanwhile, on Wednesday, suspected al-Qaida gunmen assassinated the deputy head of political security in the town of al-Shiher in eastern Hadramawt province, a security official said. In the town of Lawder in the southern province of Abyan, a security convoy was attacked by two men on motorcycles, leaving a soldier killed and three others injured, another security official said.
Yemen's 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.
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.