Police turned out in large numbers on the streets of the capital of Saudi Arabia and security is high in a number of other Gulf States, as authorities braced for protests after Friday prayers.
The demonstrations have proved decisive in popular uprisings that have overthrown the rulers of Tunisia and Egypt and are now having an impact on the oil-rich Gulf region - long thought to be largely immune to civil unrest.
Opposition groups said today they would march on the royal court in tiny Bahrain, upping the stakes in month-long demonstrations there that have cost seven lives.
Tens of thousands of anti-government protesters also marched through the capital of Yemen, the poorest country on the Arabian peninsula, and riot police fired tear gas to disperse a small protest of 200 to 300 people in Kuwait.
The new Arab awakening has even seeped into the conservative kingdom of Saudi Arabia, a long-time US ally that ensures oil supplies for the West, where a variety of online forums have sought to take grievances from the Internet to the streets.
Dozens of uniformed police patrolled the main squares in the capital Riyadh. A helicopter circled above one mosque, with busloads of police parked nearby. There was also a heavy security presence in the second city, Jeddah.
"The fact that the Saudi regime is making a big deal of this suggests that it may be a big deal," said Shadi Hamid, an analyst at the Brookings Centre in Doha.
Tension was also high in the island state of Bahrain, which is connected to Saudi Arabia by a 25-km causeway.
Bahrain's interior ministry warned that the planned march on the royal court was a threat to internal security on the island, where the majority is Shia Muslim but the ruling family is Sunni. It said its forces would intervene to head off violence.
The opposition is increasingly split in Bahrain and moderate leaders were urging hardliners to cancel the march on the royal court in Riffa, which was set to begin at 3:30pm. (12.30pm Irish time).
In Saudi Arabia, the Shia minority complain that they suffer lower living standards than Sunnis, despite the fact that many of them come from a major oil-producing region in the east of the country.
Saudi leaders have told foreign states not to interfere in their domestic affairs - a veiled warning to Shia, non-Arab Iran, often suspected of stirring discontent in the region.
"We will cut any finger that crosses into the kingdom," Saudi foreign minister Prince Saud al-Faisal said this week.
Agencies