Thousands of people protested in the Yemeni capital, Sanaa, on Tuesday to call for political reform and the resignation of President Ali Abudullah Saleh.
Inspired by recent events in Tunisia and Egypt, some of around 3,000 demonstrators clashed with pro-government supporters as they marched through the city centre.
Some protesters said police were working with militias to beat up the anti-Saleh protesters amid reports of clashes between the two camps.
Saleh became president of North Yemen in 1978 and remained in power when it merged with separatist South Yemen in 1990 after years of border clashes.
He said on February 2 that he would not seek to extend his stay in power when his current seven-year term expires in 2013.
Yemen, the Arab world’s poorest country, is a close ally of the United States in its fight against terrorism.
The US is training the Yemeni military to help stem attacks carried out by a local Al Qaeda off-shoot group.