The crackdown reflected an effort to undercut a protest movement seeking fresh momentum from the developments in Egypt, where an 18-day uprising toppled Mubarak. His ouster raised questions about the long-term stability of Yemen and other Western-allied governments in the region.
The United States is in a delicate position because it advocates democratic reform, but wants stability in Yemen because it is seen as a key ally in its fight against Islamic militants.
Hundreds of protesters had tried to reach the Egyptian embassy in Sanaa, Yemen's capital on Saturday, but security forces pushed them back. Buses ferried ruling party members, equipped with tents, food and water, to the city's main square to help prevent attempts by protesters to gather there.
There were about 5,000 security agents and government supporters in the Sanaa square named Tahrir, or Liberation. Egypt's protesters built an encampment at a square of the same name in Cairo, and it became a rallying point for their movement.
Witnesses say police, including plainclothes agents, drove several thousand protesters away from Sanaa's main square on Friday night. The demonstrators tore up pictures of President Ali Abdullah Saleh and shouted slogans demanding his immediate resignation.
Saleh has been in power for three decades and tried to blunt unrest by promising not to run again. His term ends in 2013.
Yemen is the Arab world's most impoverished nation and has become a haven for al-Qaida militants. Saleh's government is riddled with corruption and has little control outside the capital. Its main source of income - oil - could run dry in a decade.
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.