Demonstrators in Sanaa, the capital of Yemen, are calling on President Ali Abdullah Saleh to step down.
References were made to an uprising in Tunisia that ousted President Zine al-Abidine Ben Ali on 14 January and to protests in Egypt and Algeria.
Opposition members and youth activists gathered in four parts of the city, including Sanaa University, also called for economic reforms and an end to corruption.
Counter-protests were also staged on Thursday by the General People's Congress.
President Saleh has ruled the state for over 30 years. He was last re-elected in 2006.
Yemen (population 23 million) has soaring unemployment and dwindling reserves of oil and water.
There is mounting poverty and frustration with a lack of political freedoms.
Security issues include a separatist movement in the south and an uprising of Shia Houthi rebels in the north.
There are also fears that Yemen is becoming a leading al-Qaeda haven, with the high numbers of unemployed youths seen as potential recruits for Islamist groups.