By Ulf Laessing and Cynthia Johnston RIYADH (Reuters) - Small protests rattled Saudi Arabia and Kuwait on Friday, Bahrain warned that a planned rally threatened its security and Yemen witnessed huge demonstrations in continued unrest that has roiled the Arab world.
Friday rallies 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 immune to mass civil disturbances.
Police turned out in large numbers on the streets of the Saudi capital, hoping to deter a day of protests announced on Internet social media sites by a loose coalition of activists seeking political reform in the conservative kingdom.
The heavy security presence appeared to keep Riyadh quiet, but locals reported that around 200 people demonstrated in the city of Hofuf, which is close to the eastern Ghawar oil field and major refinery installations.
A similar-sized demonstration by stateless Arabs in Kuwait was met with volleys of tear gas as police swiftly dispersed the small crowd that had gathered after Friday prayers.
Some of the world's biggest oil reserves sit under the Gulf states and Saudi Arabia is the top global oil exporter. Fears that the violent unrest seen in north Africa could seep south have put markets on edge.
However, oil prices dropped on Friday, with traders betting against significant disturbances in Saudi and also concerned about the impact of a massive earthquake in Japan.
Far bigger protests rocked Yemen, with tens of thousands of pro- and anti-government demonstrators flooding many towns and cities as President Ali Abdullah Saleh struggled to maintain his 32-year grip on the Arabian Peninsula's poorest country.
"Leaving means leaving. There isn't a better option," crowds in the capital Sanaa chanted. The violence in Yemen has led to almost 30 deaths in recent weeks with neither side looking ready to concede defeat.
Hardline opposition groups in tiny Bahrain, which is connected to Saudi Arabia by a causeway, said they would march on the royal court in the capital Manama, upping the stakes in month-long demonstrations there that have cost seven lives.
Bahrain's interior ministry warned that the march was a threat to internal security on the island, where the majority is Shi'ite Muslim but the ruling family is Sunni. It said its forces would intervene to head off violence.
"The march that some people are trying to hold today to the Riffa area threatens security and social peace," it said.
No more than a few hundred to a few thousand are expected to join the Bahrain march, but politicians and activists on all sides expect Sunni civilians to come out to block their advance.
Gulf rulers are struggling to hold back a new generation of Arabs who grew up in the Internet age of easy networking and have grown increasingly bold in their demands for change.
In an act of regional solidarity, Gulf Arab oil producers on Thursday launched a $20 billion aid package for Bahrain and Oman -- a job-generating measure that will enable the two countries to upgrade their housing and infrastructure.
Youth unemployment is high across the Gulf, but protests have often focused more on gaining greater political freedom and sweeping away rampant corruption than on economic discontent.
Sectarian tensions have also come to the fore, with the Shi'ite majority in Bahrain voicing anger against the domination of the ruling Sunni dynasty.
In Saudi Arabia, the Shi'ite minority say they have lower living standards than Sunnis, despite the fact that many of them come from a major oil-producing region in the east.
Saudi leaders have told foreign states not to interfere in their domestic affairs -- a veiled warning to Shi'ite, 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.
(Reporting by bureaux across the region; writing by Crispian Balmer in Dubai; editing by Philippa Fletcher)