Tens of thousands of demonstrators, some chanting “down with the regime”, have marched in several towns and cities in Yemen against the country’s autocratic president Ali Abdullah Saleh, a key US ally in the fight against Islamic militants.
Police opened fire and tear gas to break up one of the marches, said witnesses.
Security officials confirmed a demonstrator was critically wounded by police fire. Two others were also hurt in the eastern town of Mukalla.
In the capital of Sanaa, scuffles and stone-throwing briefly erupted between thousands of anti-government demonstrators and supporters of Mr Saleh. However, police stepped in and there were no reports of injuries.
There was a heavy security presence around the interior ministry and the Central Bank buildings. Military helicopters hovered in some areas.
Anti-government protests have recently erupted in other Arab countries including Tunisia and Egypt.
In Yemen, protests erupted in several towns after Mr Saleh sought to defuse demands for his removal by pledging not to seek another term in 2013 and not to let his son inherit power.
Anti-government protesters said they did not trust Mr Saleh and demanded that he quits immediately. Supporters of the president carried banners warning that the opposition was trying to destabilise Yemen.
The United States has taken a sharp tone on Egypt, urging Mr Mubarak to move swiftly on democratic reform. But it cautiously praised reform pledges in Yemen. State Department spokesman PJ Crowley welcomed Mr Saleh’s “positive statements.”
In Yemen, where the population is overwhelmingly very young, the unemployment rate is 35% and poverty is endemic. About 40% of the population lives on less than two US dollars (£1.30) a day.