The sentencing stoked tensions in the kingdom, where small groups of demonstrators have held daily protests since emergency law was lifted on June 1, and may undermine a national dialogue planned by King Hamad bin Isa al-Khalifa to start next month.

"No dialogue with al-Khalifa! We demand the release of the prisoners," shouted some 100 protesters in one village near the capital Manama, before riot police broke up the demonstration.

In all 21 defendants, six of them tried in absentia, were charged with plotting to overthrow the government by force in collusion with "a terrorist organization" working for a foreign country. They can appeal the sentences.

Seconds after the verdict was issued, one of the defendants lined up in grey prison suits shouted: "We will continue our peaceful struggle." Other defendants responded by shaking their fists and shouting "peaceful, peaceful."

Policemen hustled them from the courtroom. Some relatives responded by chanting the Muslim rallying cry "Allahu akbar" (God is greatest) and one woman was dragged out of the chamber.

Among those who received life sentences was Shi'ite dissident Hassan Mushaimaa, leader of the hardline opposition group Haq, and Abduljalil al-Singace, from the same party. Haq joined two other groups in calling for the overthrow of the monarchy during mass protests in February and March.

Abdel Wahab Hussain, head of Wafa, another group that called for a republic, was also jailed for life. Ibrahim Sharif, Sunni Muslim leader of the secular leftist Waad party, received five years in prison. Waad and Bahrain's largest Shi'ite opposition group Wefaq had called for reform of the monarchy.

Danish-Bahraini citizen Abdulhady al-Khawaja, a rights activist, also received a life sentence, in the presence of several foreign diplomats who said the Danish embassy had not been granted access to Khawaja.

Bahrain's government said in a statement the "sentencing sends a message that law and order will be preserved.

"The defendants convicted today do not represent any significant number of the population, who actually believe that the way forward is through dialogue and peaceful means."

The United States, whose Fifth Fleet is based in Bahrain, criticized the sentences. "We are concerned about the severity of the sentences handed down," State Department spokesman Mark Toner said. "We're also concerned about the use of military courts to try these civilians."

Britain, too, expressed concern that civilians were being tried before military judges. Foreign Office Minister Alistair Burt said Ibrahim Sharif was "a prominent moderate politician who has been a constructive participant in Bahraini politics and represents a registered political party."

The U.S.-based Committee to Protect Journalists condemned as "political score-settling" the charges against the defendants, who included two bloggers.

"It is now incumbent on the international community, and particularly the kingdom's closest allies, to unambiguously convey to Bahrain that such blatant contempt for basic rights will not be tolerated," the group said in a statement.

PROTESTERS OUTRAGED

Bahrain's Sunni rulers, backed by forces from neighboring Sunni Gulf Arab states, crushed weeks of protests, mostly by majority Shi'ites, in March. Manama says the demonstrators had a

had a sectarian agenda backed by Shi'ite power Iran.

The opposition denies this, saying the protests aimed only to bring about democratic reform in the island monarchy.

Helicopters buzzed above Shi'ite villages and police armed with sound grenades and tear gas sought to snuff out protests.

In the island of Sitra, a hotspot for protests, youths blocked police cars by strewing cinder blocks, nails and wooden cupboards along village roads.

"I was furious at the nerve of the ruling family that they would give life sentences and then ask us to go to talks," said one woman protester named Maryam. "They were our symbols and the government chose to crush them. What does that say for us?"

Young men behind her regrouped in a protracted cat-and-mouse game with police. "Down, down (King) Hamad," they shouted.

Some observers have suggested King Hamad may try to cool tensions before the dialogue by granting a general amnesty to many of those jailed in recent trials.

In a statement, Wefaq said "these sentences will create an eternal political crisis without a quick ... solution."

Earlier, Wefaq spokesman Khalil al-Marzouq said Wefaq would not meet the government's Thursday deadline for responding to the invitation to dialogue, and could not say if it would join.

"Those people are a critical portion of the movement. How can there be a dialogue while they are in prison?" he asked.