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.