Colombo - Voting in a crucial provincial election in eastern Sri Lanka opened Saturday amid high security and under the watch of more than 4,000 observers, officials said.

The election is being held in three districts - Trincomalee, Batticaloa and Ampara - where nearly 1 million people are eligible to vote in the region's first poll in 20 years to elect a total of 37 members to the provincial council.

Polling stations opened at 7:00 am (0130 GMT) and close at 4:00 pm.

The election is being held after government troops recaptured rebel-held areas in the eastern province during a one-year long operation last year.

Ahead of the poll, suspected Tamil rebels set off a powerful bomb explosion in the Ampara town, 320 kilometres east of the capital, killing 12 civilians and injuring 35 others on Friday evening.

Voting in the Ampara area, an area that is majority Sinhalese, could be affected due to the incident but the attack will not affect the overall situation, poll observers said.

In a second incident a merchant vessel used by the Navy was sunk in an underwater explosion in the Trincomalee harbour, 240 kilometres north east of the capital.

The election is being closely competed between the ruling United People's Freedom Alliance (UPFA), led by President Mahinda Rajapaksa, and the main opposition United National Party (UNP) headed by former prime minister Ranil Wickremesinghe.

Rajapaksa is expecting voters to endorse his military campaign of recapturing the east, while the opposition's main argument has been that a breakaway group of rebels are tied to the ruling party, are still carrying weapons and pose a threat to civilians.

Reports from the area said that enthusiasm among voters was high and a strong turnout was expected.

More than 30,000 police officers have been deployed for security and 16,000 officials to conduct the poll.

Election results were expected to start coming in shortly after midnight and the final results by Sunday afternoon. (dpa)