* Security official suspects blast was "terrorist attack"
* British Foreign Office urges its citizens to leave Yemen
* In Sanaa, opposition leader survives assassination attempt (Adds UK urging Britons to leave Yemen, opposition statement on assassination attempt)
By Mohammed Mukhashaf and Mohammed Ghobari
ADEN/SANAA, July 20 (Reuters) - A blast in a booby-trapped car killed a Briton living in Yemen's southern port city of Aden on Wednesday, officials said, and the British government urged its citizens to leave the country.
Opposition officials said one of their political leaders survived an assassination attempt in the capital Sanaa and warned of further unrest.
A security source told Reuters he believed militants were behind the blast that killed the long-time British resident of Aden, who was in his 60s, as he started his car.
The victim was an independent surveyor working for marine and insurance compan ies in Yemen. A Western shipping source based in Aden said the Briton had just returned from surveying a tanker attacked in July by pirates off Yemen's coast.
"We tend to think that it was some kind of terrorist attack because he was well known," the security source said.
Attacks on foreigners are rare in the port city, which lies east of a strategic shipping lane through which some 3 million barrels of oil pass daily.
The impoverished Arabian Peninsula state, neighbour to Saudi Arabia, is grappling with growing turmoil as mass protests calling for an end to President Ali Abdullah Saleh's rule drag into a sixth month. Washington and Riyadh fear the unrest gives al Qaeda's Yemen-based wing more room to operate.
Witnesses in Aden said the British victim's car blew up as soon as he turned on the engine, shattering windows in nearby buildings on Aden's Mualla Plaza.
"He started the car and it immediately exploded and he was engulfed in flames," a witness said by telephone.
One passer-by was critically wounded in the blast which also damaged nearby buildings, a municipal official said. She said there was no indication who was behind the explosion.
"It's clear that there was some kind of explosive device placed in his car," she said.
A British Foreign Office spokeswoman confirmed the Briton's death and said the government was advising all British nationals against travelling to the whole of Yemen, a position which "couldn't be any firmer" .
Aden had been relatively quiet in recent months, even as protests in other parts of the country erupted into sporadic violence.
The neighbouring Abyan province has witnessed daily bloodshed since Islamist militants seized the city of Jaar in March and the provincial capital of Zinjibar in May.
The army, which says the militants are part of al Qaeda's Yemen-based wing, launched an offensive to retake Zinjibar five days ago but has yet to regain the city.
The army impose a security belt around Aden several weeks ago to try to prevent more militants from slipping into the strategic port city. It had previously reported that several al Qaeda linked operatives had snuck into Aden, where some 54,000 Abyan residents have sought refuge.
Further north in the capital Sanaa, opposition officials said the head of the leading Islamist party Islah survived an attack by gunmen.
Sanaa has been the scene of rising violence in recent days following weeks of relative calm after Saleh travelled to Saudi Arabia for treatment on wounds he suffered in a bomb blast at his compound.
Yemen's opposition coalition said the gunmen fired on Islah leader Mohammed al-Yadumi's car but he escaped the attack. The group blamed Saleh's family and forces loyal to him, and warned the move could spark worse violence in the fragmented nation.
" Targeting the head of Islah is an attempt to plunge the country into civil war . Saleh's family is deluded in thinking it can remain in power ," the coalition said in a statement. (Reporting by Mohammed Mukhashaf in Aden, Mohammed Ghobari in Sanaa, Humeyra Pamuk in Dubai and Stephen Mangan in London; Writing by Erika Solomon; Editing by Alistair Lyon)