SANAA, Yemen - Tens of thousands of protesters Thursday staged unprecedented demonstrations against Yemen's autocratic president, a key U.S. ally in battling Islamic militants, as unrest inspired by uprisings in Egypt and Tunisia spread further in the Arab world.

The West is particularly concerned about instability in Yemen, home of the terrorist network al-Qaida in the Arabian Peninsula. U.S. counterterrorism officials are worried that Yemeni security forces will be more focused on protecting the government, allowing al-Qaida to take advantage of any diminished scrutiny.

President Ali Abdullah Saleh, in office for more than three decades, announced Wednesday he would not seek re-election in 2013 and would not seek to pass power to his son. Saleh's pledge was seen as an attempt to defuse growing calls for his ouster.

Opposition groups said they are suspicious of Saleh's offer, however, and want concrete proposals for change.

On Thursday, they led tens of thousands in protests in seven towns and cities across Yemen, with chants of "Down, down, down with the regime!" and banners calling on the president to resign now.

In the capital of Sanaa, several thousand government supporters staged a counterdemonstration, carrying banners warning that the opposition is trying to destabilize Yemen. Military helicopters hovered in some areas, and there was a heavy security presence around the Interior Ministry and the Central Bank.

The marches were largely peaceful, although witnesses said police opened fire in one provincial town, critically wounding a protester. In the capital, scuffles and stone-throwing briefly erupted between government supporters and opposition marchers, but police stepped in and there were no reports of injuries.

The Obama administration has cautiously praised Saleh's offer of reform, in contrast to the sharp tone on Egypt, where President Hosni Mubarak is trying to cling to power until September, despite demands delivered in 10 days of massive protests that he leave office immediately.

The White House said President Barack Obama called Saleh and urged him to follow through on his pledge to reform his government, and asked that Yemeni security forces refrain from violence against protesters.

U.S. State Department spokesman P.J. Crowley welcomed Saleh's "positive statements" about including opposition elements in a reform process, but said that "it is important for governments across the region ... to follow statements with actions."

Saleh is a weak but increasingly important partner for Washington.

Yemen has become a main battleground against al-Qaida. The government, which receives millions of dollars in U.S. military aid, has allowed American drone strikes on al-Qaida targets and has stepped up counterterrorism cooperation.

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 December 2009 bombing of a Detroit-bound airliner and the unsuccessful plot to send mail bombs on planes from Yemen to the U.S.

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.

Yemen is the poorest Arab country, with nearly half the population living below the poverty line of $2 a day.

Foreign Minister Abu Bakr al-Qirbi acknowledged Thursday that frustration of the young generation is widespread across the Arab world, including in his country. But he warned that interference from outside countries - he mentioned Iraq, Afghanistan or Pakistan - would be counterproductive.

Speaking in Brussels, where he sought development aid, al-Qirbi argued that Yemen's government was better placed to hold constructive internal dialogue. He said Yemen's leaders never severed contacts with opposition parties and civil groups.

However, Yemen's opposition groups said they don't trust the government's promises.

While some opposition figures have expressed readiness for dialogue, demands could harden as protests continue, said Mohammed al-Sabri, a spokesman for a coalition of opposition groups.

"We will be able to answer the call of the people, regardless of what it is, including their slogans of ending the regime and pushing out the leader," al-Sabri said.

The anti-government demonstration in Sanaa brought together young people, workers and women in black robes who initially planned to gather in downtown Tahrir (Liberation) Square, the same name of the site in downtown Cairo where Egypt's protesters have gathered daily since Jan. 25.

However, government supporters, including civil servants, pre-empted the protesters, taking control of the square first. One of the demonstrators, unemployed Iyad Nasser, said he and others had been paid to show up.

Several dozen of Saleh's supporters headed to where the anti-government protesters had assembled, near Sanaa's university, and scuffles erupted. Police separated the two camps.

In the city of Aden, thousands of marchers defied security forces and armored personnel carriers that tried to close the main streets to prevent them from gathering. Protesters also scuffled with security forces in the town of Jaar in the southern province of Abyan, where al-Qaida militants have been active.

Yemen's government is riddled with corruption. Tens of thousands of residents have been displaced from their homes by conflict. The country is wrestling with a lingering rebellion in the north and a secessionist movement in the south.

With the Arab world transfixed by Egypt's uprising, dissatisfaction with corruption, unemployment and poverty is beginning to rise to the surface elsewhere.

Jordan has been the scene of several large rallies against rising prices, and King Abdullah II sacked his Cabinet this week, bowing to public pressure. The Muslim opposition and their leftist allies are expected to stage more protests outside the prime minister's office's Friday, repeating their call for the newly appointed premier to step down. But the numbers are expected to be smaller than usual because some political groups said they would not participate in order to give Marouf al-Bakhit the chance to form his Cabinet.

In Algeria, opposition leaders, human rights groups, unions, students and jobless workers are planning a march next week, despite a ban by the government. One of the protesters' key demands is to lift a state of emergency in effect since 1992 when the country spiraled into a civil war between Islamists and government forces. Algeria's state news agency on Thursday quoted the president as saying the state of emergency would be lifted in the "very near future."

Syrians have been organizing online campaigns for a "day of rage" in Damascus on Friday and Saturday. More than 13,000 people have joined a Facebook page calling for "the Syrian Revolution 2011," although many of the members are believed to live outside the country.