MOGADISHU (Reuters) - Somalia's al Shabaab Islamist militant group has called on its followers to kill clan elders, officials and lawmakers taking part in the parliamentary election and to attack polling venues.

Voting for the 275-seat parliament starts on Sept. 25 and ends on Oct. 10. Lawmakers will be chosen by clan elders and sworn in on Oct. 30. The lawmakers will then choose a president.

Somali authorities and international sponsors had to scrap a plan for each person to get a vote, largely due to security challenges as the Western-backed government is still battling the Islamist insurgency. [nL8N1AK0OJ]

Al Shabaab, which frequently attacks government targets, wants to topple the government of President Hassan Sheikh Mohamud, who is seeking re-election.

"Attack all places where election preparation goes on... Kill anyone who participates in the so-called election," Ali Mohamud Rage, the group's spokesman, said in audio posted online late on Monday.

The election is part of efforts to rebuild a nation that has been shattered by two decades of war and chaos. Rage said it was not legitimate as it was backed by non-Muslims.

"Somalia's election will take place as planned without fear," said Abdifatah Omar, spokesman for the local government of Mogadishu, adding security had been tightened in the city.

(Reporting by Feisal Omar and Abdirahman Hussein; Writing by Duncan Miriri; Editing by Janet Lawrence)