Thousands of Shiites took to the streets on Friday to demonstrate against a proposal to unite the Sunni-ruled kingdom with neighbouring Saudi Arabia, also a Sunni monarchy, witnesses said.
"No to union. Bahrain is not for sale," protesters chanted as they marched along a main road linking a number of Shiite villages around the capital.
Others shouted "the country is not up for auction," as the crowds responded to a call by the predominantly Shiite opposition to march under the slogan "ready to sacrifice (ourselves) for Bahrain."
Shiite spiritual figure Ayatollah Issa Qassem, speaking at a mosque in Duraz, west of Manama, demanded that any proposed union of the two countries be submitted to a referendum.
The principal opposition group, Al-Wefaq, has made the same demand.
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The people have the "right to oppose or approve this union," Qassem said.
"Why prevent the people from expressing their fear, their opposition and their opposition and their legitimate peaceful resistance to a project that is being forced on them?" he asked.
The six members of the Gulf Cooperation Council, a grouping of the Sunni Arab monarchies in the Gulf, have been discussing a Saudi proposal that would lead toward some form of closer political union.
The first step in this process would be the union of Bahrain, which has a Shiite majority, with Saudi Arabia.
In predominantly Shiite Iran, just across the Gulf, thousands of people demonstrated on Friday to protest the proposed union, which the authorities have called an "American plan to annex Bahrain to Saudi Arabia."
Tensions have escalated between Iran and its Gulf Arab neighbours since a Saudi-led Gulf force rolled into Bahrain in March 2011 to boost the kingdom's security forces, which then crushed a month-old Shiite-led uprising.