DUBAI, United Arab Emirates — Bahraini protesters poured back to the streets Wednesday after a security court sentenced eight Shiite activists to life in prison in the latest blow by the Western-backed kingdom to cripple the biggest Arab Spring opposition movement in the Gulf.

The fast and angry reaction to the verdicts — the most significant display of unrest in weeks — underscored the volatility in the island nation after four months of unrest and raised questions about whether any credible pro-reform leaders will heed calls by the Sunni monarchy to open talks next week.

In size, Bahrain is little more than a speck off the coast of Saudi Arabia. But it draws in some of the region’s major players: hosting the U.S. Navy’s 5th Fleet and serving as a growing point of friction between Gulf powers Saudi Arabia and Iran.

Security forces used tear gas to drive back hundreds of Shiite marchers trying to reach a central square in the capital Manama, which was once the hub of their protests for greater rights. In other Shiite areas, protesters gathered in the streets but were held back by riot police. No injuries were reported.

Bahrain has allowed two major rallies this month by the main opposition party, but the confrontations Wednesday were among the biggest challenges to security forces since martial law-style rule was lifted June 1.

Shiites account for 70 percent of Bahrain’s population of some 525,000, but claim they face systematic discrimination such as being barred from top government and political posts.

Bahrain’s government said in a statement late Wednesday that the convicted activists were responsible for “bringing the country to the brink of total anarchy” with a wave of marches and sit-ins earlier this year.

The official Bahrain News Agency said those sentenced to life include prominent Shiite political figures Hassan Mushaima and Abdul Jalil al-Singace and rights activist Abdulhadi al-Khawaja. Mushaima returned from self-exile in London earlier this year after Bahrain’s leaders promised to erase old charges of opposing the state.

Eight received life sentences while 13 others received shorter prison terms apparently because they weren’t considered leaders.

Fourteen of the 21 convicted are in custody while the rest were sentenced in absentia by the security court, which uses military prosecutors and a military-civilian tribunal. Among the life sentences, however, all but one of the suspects was in Bahrain.