Akhbar Shakuri and Mohammed Filfel died and a dozen other protesters were wounded during the clashes that erupted when police opened fire to disperse a demonstration against the arrest of Sheikh Nimr al-Nimr, said the activists.
The violence occurred in Riyadh Street, the main artery of Qatif city, they said. The reports could not be independently verified.
The interior ministry described Nimr as an "instigator of sedition" as it announced that he was arrested at Al-Awamiya in Eastern Province on Sunday, after being wounded in the leg while putting up resistance.
He was transferred to hospital and was due to be interrogated, ministry spokesman Mansur Turki said, cited by the official SPA news agency.
The new deaths bring to nine the number of people killed in clashes between Saudi authorities and protesters in the Shia-populated region.
Nimr is considered one of the main advocates of demonstrations that first took place in February 2011 after an outbreak of violence between Shia pilgrims and religious police in the holy city of Medina.
The protests escalated after the kingdom led a force of Gulf troops into neighbouring Bahrain to help crush a month-long Shia-led uprising against the country's Sunni monarchy.
Most of Saudi Arabia's estimated two million Shias live in the east, where the vast majority of the OPEC kingpin's huge oil reserves lie. Saudi Shias complain of marginalisation in the kingdom.
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I request Shias that please never forget that Saudi Arabia is not like Pakistan where you shall be fully allowed to do any thing in any forms and manifestations and where you shall be provided foolproof security blanket including jamming the mobile services.......
@Ali Wali: Following from that I am the richest person in the world, the first person to step on the moon was Javaid Miandad and Veena Malik has started doing hijab.
Look and Identify the opressor and oppressed.
It's pretty much calm down here. Why even publish such news when you yourself mention that it could not be independently verified.
With such brutal killings and oppression by the state towards its citizens, Saudi Arabia cannot remain immune from the Arab spring. The youth of Saudi Arabia have the same rights and aspirations as their Arab brothers and sisters and Washington may have to cut another dictator loose.