The island nation of nearly 800,000 people is also crucial to U.S. interests in the region: It hosts the U.S. Navy's 5th Fleet.
The demands of protesters have grown over three days, with the crowd torn between those who want constitutional reforms and others who now say openly that they want the family of the king to step down.
Bahrain's protest movement appears to be largely leaderless, although medical and media centers have been organized by the demonstrators. Some credit the use of Facebook, Twitter and blogs with providing a forum for activists to trade ideas and promote the rally.
Before Thursday's crackdown, the unrest in Bahrain had emerged among the most potent anti-government movements in a string of sometimes lethal demonstrations sweeping the Middle East, importing the lessons of North Africa's recent uprisings into the oil-rich Persian Gulf region.
The successful uprisings in Tunisia and Egypt have become models for a daily phenomenon in places such as Yemen and Bahrain. With new marches planned in several other countries over the next several days, including Morocco and Libya, governments from North Africa to the gulf were settling in for what looks to be an extended period of instability.
One of the more volatile fronts is Libya. Anti-government protests erupted in several towns there Wednesday, with reports of police stations burned in Qubbah, Zentan and Baida, and overnight clashes in Benghazi on Tuesday.
The most serious reports of violence in Libya remained unconfirmed because the government of Moammar Kadafi imposes heavy restrictions on the operation of journalists.
Opposition activists reported on social media that police had opened fire with live ammunition on demonstrators in Baida and Benghazi. At least five people reportedly died in the violence Wednesday, which broke out a day before major street demonstrations were planned for the North African nation.
Street fighting in Benghazi began Tuesday night, when protesters gathered in response to the arrest of a well-known human rights lawyer became agitated over reports of a fire in Abu Selim military prison. Many outside the prison were awaiting the release of family members, Libyan journalist Fatthi Ben Eissa said in a telephone interview.
"It started out with a few tens of families. That became 300 demonstrators in a few minutes, and very quickly escalated to 2,000," said Mohammed Ali Abdallah, deputy secretary-general of the exiled opposition group the National Front for the Salvation of Libya.
Protest organizers have called for a "Day of Rage" across Libya on Thursday.
Abdallah said his group was urging the departure of Kadafi and the institution of a constitutional form of government, but he said the widening protests in the country were not associated with any organized political faction.
"The regime will go all out to repress this revolution. But the inspiration the people have taken from Tunisia and Egypt, it's really broken down the wall of fear that's kept many people afraid for so many years," he said.
On Wednesday, protests in Yemen spread to the southern port city of Aden, where police opened fire in an attempt to break up a surging crowd, killing two.
Al Jazeera television reported that about 500 protesters hurled stones at police, set cars on fire and stormed a municipal building before gunshots rang out and tear gas was fired.
In Sana, the capital, a sixth straight day of protests turned violent Wednesday in another round of clashes between about 800 students seeking the ouster of President Ali Abdullah Saleh and young people loyal to the regime who were armed with batons, stones and daggers.
At least four people were wounded, Reuters news agency reported.
In Iran, government supporters paraded through the streets bearing the coffin of university student Saane Zhaleh, 21, killed Monday during a march by tens of thousands of Iranians seeking the ouster of the theocratic regime.
The government describes Zhaleh as the "martyr Basij" and says he was killed by government opponents when he joined citizen Basiji militiamen to help put down the protests, which were prohibited by the government.
The opposition says he was a supporter of the anti-government protesters and was shot by police.
State television showed marchers carrying Iranian flags and shouting slogans against opposition leaders Mehdi Karroubi and Mir-Hossein Mousavi, including "Death to Karroubi!" and "Death to Mousavi!"
There were brief clashes between the dueling sides but no injuries, according to state TV.
Iranian officials also confirmed the death of a second "passerby" at Monday's protests, 22-year-old Mohammad Mokhtari, who, according to the semiofficial Fars News Agency, was "wounded by a number of rioters."
In Jordan, the latest in a series of largely peaceful protests was staged in the rain by about 150 people outside one of King Abdullah II's palaces, where labor activists and teachers demanded a return to a constitution that more evenly divided power between the king and the parliament.
Parker reported from Manama and Murphy from Amman. Special correspondent Meris Lutz in Beirut and Amro Hassan of The Times' Cairo bureau contributed to this report.