Thousands of mainly Shia Bahrainis have protested outside the capital against a plan to create a union of Gulf Arab countries.
In Manama, the demonstration stretched for some three miles (5km), blocking a motorway.
The Saudi king's proposal to unite the six members of the Gulf Co-Operation Council was discussed this week.
The six members are Bahrain, Kuwait, Oman, Qatar, Saudi Arabia, and United Arab Emirates.
There were also protests against the move in Dubai and Iran, where Bahrain's ambassador was rebuked.
The planned union between Bahrain and Saudi Arabia - the first part of a broader union of the Gulf's six Arab monarchies - has triggered discord between Shia-dominated Iran and the Sunni rulers of Shia-majority Bahrain.
'Not for sale'
Iran has called the proposal "the American plan to annex Bahrain to Saudi Arabia".
In Tehran, demonstrators took to the streets, many brandishing the Bahraini flag.
The Iranian authorities also reportedly summoned Bahrain's ambassador after the country's Foreign Minister Sheikh Khaled warned Iran to stop meddling in its internal affairs.
Reports quoting an Iranian foreign ministry official said that Iran "rejects comments made by the Bahraini foreign minister and hopes that the Bahraini government finds a suitable solution towards the developments there".
Meanwhile, in Dubai, thousands protested, chanting: "No to union. Bahrain is not for sale!" as they marched along a main road linking a number of Shia villages around the capital.
After holding a meeting on the proposal earlier during the week, Gulf leaders have decided to delay any decision on the unity plans.
Dozens of people, mainly Shia, have been killed in pro-democracy protests in Bahrain, since February 2011.
Tensions between Iran and Bahrain were further stoked when a Saudi-led Gulf force entered the country in March 2011 to bolster the kingdom's security forces and crushed the uprising.