KUWAIT, May 19 (Reuters) - Kuwait will deport 15 Egyptians who took part in an illegal rally in support of Egyptian presidential candidate Abdel Fattah al-Sisi, a Kuwaiti government source said.

Kuwaiti authorities are especially wary of political gatherings that could stir tensions in the Gulf Arab state, where expatriates make up around two-thirds of the 3.8 million population.

Foreigners are not allowed to hold political events and there are restrictions on public rallies held by Kuwaitis.

The source, who declined to be named, did not give further details.

Kuwaiti newspaper al-Watan said on Sunday that the 15 were travelling by bus south of the capital on Saturday. They were seen beating drums and waving, the report said, and planned to join other Egyptians to express their support for Sisi before they were arrested by police.

Egypt's presidential election is being held on May 26-27 and overseas voters have already started going to the polls. Officials at the Egyptian Embassy in Kuwait, which has been open for voting since Thursday, were not immediately available for comment.

Major oil producer Kuwait welcomed the military's overthrow of Egypt's Islamist president Mohammed Mursi after mass protests and sent billions of dollars in financial aid to shore up the ailing economy.

But like other Gulf Arab states it does not tolerate foreigners participating in politics in public. Last year it deported nine Egyptians who demonstrated against the army crackdown on the Muslim Brotherhood, of which Mursi is a leading member.

(Reporting by Ahmed Hagagy, Writing by Sylvia Westall; Editing by John Stonestreet)