SANAA, Yemen — Tens of thousands of demonstrators, some chanting "down, down with the regime," marched Thursday in several towns and cities in Yemen against the country's autocratic president, a key U.S. ally in the fight against Islamic militants.

Police opened fire and tear gas to break up one of the marches, witnesses said, and security officials confirmed a demonstrator was critically wounded by police fire. Two others were also hurt in the eastern town of Mukalla, but further details were not immediately available.

In the capital of Sanaa, scuffles and stone-throwing briefly erupted between thousands of anti-government demonstrators and supporters of President Ali Abdullah Saleh, who has ruled for more than 30 years. However, police stepped in and there were no reports of injuries.

Security forces deployed in large numbers around the Interior Ministry and the Central Bank, and military helicopters hovered over some parts of the city.

Anti-government protests have erupted in several Arab countries in recent weeks. In Egypt, embattled President Hosni Mubarak is trying to cling to power until the end of his term in September, despite more than a week massive street protests demanding his immediate resignation.

In Yemen, protests erupted in several towns Thursday after Saleh moved earlier this week to defuse demands for his ouster by pledging not to seek another term in 2013 and not to allow his son inherit power.

Anti-government protesters said they don't trust Saleh and demanded that he quit immediately.

"Thirty years of promises and thirty years of lies," read one banner raised by marchers in Sanaa. Protesters chanted: "Down, down with the regime."

In a counter demonstration, supporters of the president carried banners warning that the opposition was trying to destabilize Yemen.

Mohammed al-Sabri, a spokesman for a coalition of opposition groups, said hundreds of thousands took to the streets Thursday. He said the opposition is ready to engage in a dialogue with the president, but wants concrete proposals for change.

"We welcome this decision (not to seek another term), but if the people want the president to leave, we will adopt their demand," al-Sabri said. "We have had political demands which we discussed with the regime for the past three years, but unfortunately failed."

He said peaceful protests would continue for the next three months.

The United States has taken a sharp tone on Egypt, urging Mubarak to move swiftly to meet the demand for democratic reform. But it has cautiously praised pledges of reform in Yemen.

State Department spokesman P.J. Crowley on Wednesday welcomed Saleh's "positive statements."

The Yemeni president is seen as a weak but increasingly important partner of the United States, allowing American drone strikes on al Qaeda targets and stepping up counterterrorism cooperation.

In Brussels, Yemen's foreign minister, Abu Bakr al-Qirbi, warned that interference from outside countries -- of the sort in Iraq, Afghanistan or Pakistan -- would be counterproductive.

He said frustration of the young generation was widespread across the Arab world, including in his country. "I think the frustrations of younger generations are universal in the Arab world," al-Qirbi told The Associated Press in Brussels, where he had come to seek development aid.

However, he said Yemen's government never severed contacts with opposition parties and civil groups, and for that reason it was better placed to hold a constructive internal dialogue than other countries in the Middle East.

In Yemen, where the population is overwhelmingly very young, unemployment is 35 per cent and poverty is endemic. About 40 per cent of the population lives on less than $2 (euro1.45) a day.

Saleh's government controls little of the impoverished country beyond the capital; it is facing a serious challenge from a secessionist movement in the south and a rebellion in the north.

The U.S.-born radical cleric Anwar al-Awlaki, thought to be hiding in Yemen, is believed to have inspired and even plotted or helped coordinate recent attacks on the U.S. Those include the failed Christmas Day 2009 bombing of a Detroit-bound airliner and the unsuccessful plot to send mail bombs on planes from Yemen to the U.S. in October.

Al-Awlaki also is believed to have inspired the deadly 2009 shooting rampage at Fort Hood, Texas, and had ties to some of the 9/11 hijackers.

In Thursday's marches, thousands of anti-government protesters also took to the streets in the city of Aden. They defied security forces and armored personnel carriers that tried to close the main streets to prevent them from gathering.

Protesters there shouted: "People want the downfall of the regime, the downfall of the president."

All big shops in Sanaa and Aden closed their doors and major companies hired guards to protect against possible looting.

Protesters also scuffled with security forces in the town of Jaar in the southern province of Abyan, where al Qaeda militants have been active.