A Saudi court on Wednesday sentenced prominent Shiite cleric and anti-government protest leader Nimr al-Nimr to death after convicting him of sedition, his family said.
The verdict risks a further escalation of tensions with Saudi Arabia's minority Shiite community.
Nimr was a driving force behind demonstrations against the Sunni authorities that erupted in 2011 in the oil-flush east, alongside a Shiite-led protest movement in neighbouring Bahrain.
The cleric was also convicted of seeking "foreign meddling" in the country, a reference to Iran, his brother Mohammed al-Nimr wrote on Twitter.
He was found guilty of "disobeying" the kingdom's rulers and taking up arms against security forces, his brother said.
Nimr's family accused the court of ignoring the sheik's "peaceful and non violent approach," saying the case had caused "social and political discontent".
"The ruling... is political par excellence," his relatives in the Eastern Province communities of Awamiya and Qatif said in a statement.
They called for a dialogue with officials, "out of concern for our dear country".
Nimr, who is in his 50s, had been on trial since March 2013.
Amnesty International slammed the verdict as "appalling," dismissing Nimr's trial as "deeply flawed," and calling for the sentence to be immediately quashed.
The sentence is part of a campaign by the authorities "to crush all dissent, including those defending the rights of the Kingdom's Shiite Muslim community," said Amnesty's Said Boumedouha.
Nimr "must be released and Saudi Arabia must end its systematic discrimination and harassment of the Shiite community," he said in a statement.
A diplomatic source said the sentence would trigger " a lot of anger".
But whether that would translate into renewed street protests was hard to say, said the source, requesting anonymity.
Most of Saudi Arabia's estimated two million Shiites live in the east, where the vast majority of the wealthy kingdom's oil reserves lie, and many complain of marginalisation.
They began demonstrating in February 2011 after an outbreak of violence between Shiite pilgrims and religious police in the Muslim holy city of Medina in western Saudi Arabia.
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- 'By peaceful means' -
Protests escalated after the kingdom's intervention in neighbouring Bahrain to support a Sunni monarchy against an uprising led by the Shiite majority.
Hundreds were arrested in Saudi Arabia, according to Amnesty.
Tension rose further in July 2012 when security forces arrested the grey-bearded Nimr, who was shot and wounded.
The kingdom's former interior minister, Prince Ahmed bin Abdul Aziz, described Nimr as mentally ill.
Activists said early this year that the cumulative toll from clashes had reached 24, including at least four policemen.
In June a court sentenced two people to death for "taking part in forming a terrorist group" and other crimes linked to the protests by Shiites.
Several others have received multi-year jail sentences.
Speaking in November 2011, after four Shiites were shot dead in Eastern Province, Nimr had demanded the "release of all those detained in the protests, and all prisoners of conscience -- Sunnis and Shiites".
In a speech at the funeral of one of the protesters, Nimr said: "We are determined to demand our legitimate rights by peaceful means."
Tensions eased in August 2012 after seven Shiite dignitaries from Qatif hailed a call by King Abdullah for the creation of a centre for Sunni-Shiite interfaith dialogue.
But discontent has continued to simmer in the east.
Police said on September 30 that one of their officers was wounded when gunmen opened fire on a checkpoint in the Shiite village of Awamiya.
Two days earlier a suspect was shot and killed during a firefight with police in the same village.