Police have flooded the Yemeni capital of Sanaa in an attempt to end days of protests against the president of 32 years, a key Western ally in battling al Qaida.
The officers fired in the air and blocked students at Sanaa University from joining thousands of other protesters who were holding a sixth straight day of demonstrations.
Taking inspiration from the toppling of leaders in Egypt and Tunisia, Yemen's protesters are demanding political reforms and the removal of president Ali Abdullah Saleh. Their central complaints are poverty, unemployment and corruption.
Yemen is a conflict-ridden and impoverished nation. Its president has become a crucial partner in battling al Qaida in the Arabian Peninsula, the terror network's offshoot in Yemen.
The group's several hundred fighters have battled Mr Saleh's US-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 US military plans a 75 million dollar (£46.7m) training programme with Yemen's counter-terrorism unit to expand its size and capabilities in the nation's difficult mountain terrain. It is a difficult balancing act for Mr Saleh, who has been criticised as being too close to the US
Now facing unprecedented street demonstrations, he has tried to defuse protesters' anger by saying he will not run for another term in elections in 2013 and that he will not seek to set up his son to succeed him.
However protesters chanted slogans against the president's son, Ahmed.
Police chained the university's iron gates shut to prevent students from pouring into adjacent streets. At least four protesters were wounded in scuffles.
Demonstrations are also taking place in the port city of Aden and in Taaz, where thousands of protesters shouted, "Down ... down with Ali Abdullah Saleh."