Witnesses say Bahrain security forces have fired tear gas at protesters after a security court sentenced eight Shiite activists to life in prison.

Witnesses say marchers were trying to reach a central square in Bahrain's capital Manama, which was once the hub of their protests for greater rights in the Gulf kingdom. It was the first significant effort by protesters in months to return to the streets.

The court Wednesday issued life sentences against eight Shiite activists and long jail terms for 13 others after convictions on seeking to overthrow the ruling system. Fourteen of the defendants were in custody, and the others were sentenced in absentia.

Bahrain's majority Shiite claim widespread discrimination by the Sunni monarchy.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

MANAMA, Bahrain (AP) _ A security court sentenced eight Shiite activists to life in prison Wednesday and issued long jail terms for 13 others in the latest blow by authorities waging a crackdown against protesters seeking greater rights in the Gulf kingdom.

Shiite crowds blocked roads with sand piles and called for protest marches _ suggesting the court decision could spark another round of unrest after months of a security lockdown by military and police units in Bahrain, a key American military ally that is home of the U.S. Navy's 5th Fleet.

The wife of one of the activists said he shouted "Our people demand freedom" as the verdicts were read.

The sentences also could imperil U.S.-backed efforts by Bahrain's Sunni leaders to open talks next week with Shiite groups, which have demanded an end to the political trials and withdrawal of a Saudi-led regional force helping prop up the Bahrain ruling family.

"We should conduct the dialogue in an open atmosphere, not when people are being arrested," said Khalil al-Marzooq, who was among the 18 Shiite parliament members who staged a mass resignation to protest the crackdowns earlier this year.

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

The protests _ inspired by the wider Arab uprisings across the Middle East _ have claimed at least 31 lives since February and put U.S. officials in the difficult position of both denouncing the violence but standing by the allied rulers.

The Shiite activists were charged with trying to overthrow Bahrain's 200-year-old monarchy and of having links to "a terrorist organization abroad."

Bahrain's rulers fear that any gains by Bahrain's Shiites could open new footholds for influence by Iran, a predominantly Shiite country that is a main regional rival of the Sunni Arab-led nations just across the Gulf. Bahrain also accuses Iranian-backed Hezbollah of having a role in the protests.

Shiite leaders in Bahrain have repeatedly denied any ties to Iran and accuse leaders of using the fears of Iranian string-pulling to wage crackdowns that have included hundreds of arrests and purges from jobs and universities.

Fourteen of the 21 convicted are in custody while the rest were sentenced in absentia by the security court, which used military prosecutors and a military-civilian tribunal. Among the life sentences, however, just one was against an activist outside the country.

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 wipe away old charges of opposing the state.

Pro-reform activist Ibrahim Sharif _ the only Sunni among the suspects _ received five years in prison while other sentences ranged from two to 15 years. The sentences can be appealed.

Sharif's wife, Farida Ghulam, said her husband cried out "Our people demand freedom" after the sentences were read.

Ghulam said al-Khawaja then shouted: "We will continue our struggle." His daughter, Zainab, was dragged from court by female guards after she yelled "Allahu akbar," or "God is great," said Ghulam.

"It's a political verdict," said Nabeel Rajab, president of the Bahrain Center for Human Rights. "All those convicted today were targeted because of their activities to bring about change and democracy in Bahrain."

The verdicts could also bring some direct diplomatic fallout. At least two of those sentenced to life also hold European passports: al-Khawaja, who is a Danish citizen, and Mohammed Habib al-Muqdad, who has Swedish citizenship.

The court was set up during martial law-style rule that was imposed in March and lifted June 1. It has presided over a series of cases linked to the protests, including issuing two death sentences against men accused of killing police officers during the demonstrations.

Next week, a trial is scheduled to resume for more than 30 doctors and nurses accused of supporting the protests. Some of the medical personnel claim they were abused while in custody.

The Ireland-based rights group Front Line condemned the verdicts and the use of military prosecutors.

The group's executive director, Mary Lawlor, said it "underlines the determination of the government of Bahrain to secure a conviction at any cost."

Hours before the court session, the state news agency announced that the government had sent out 300 invitations to political parties and individuals for the proposed dialogue to begin July 1. The report does not name any of the potential participants.

"The dialogue aims at bringing together the various segments of the Bahraini society to present the people's views and demands for further reform in the country," the report quoted Parliament Speaker Khalifa bin Ahmed al-Dhahrani, who is in charge of the talks.