(Adds al Shabaab comment on the election)

By Abdi Sheikh and Feisal Omar

MOGADISHU Feb 7 (Reuters) - Somali authorities blocked main roads in the capital and barred vehicles from driving near the secure airport compound on Tuesday in a security lock down before a presidential election.

After months of delays, 329 newly sworn-in members of parliament will on Wednesday choose whether to back President Hassan Sheikh Mohamud for a second term or one of 21 rivals.

Rival candidates have accused each other of vote-buying, a practice in past votes Western donors have sought to stamp out.

Diplomats say corruption continues to hamper efforts to rebuild after years of conflict, while the government is also battling an Islamist insurgency.

"Police forces will secure the election scene and streets, and the vote will take place peacefully as planned," Mohamed Sheikh Hassan Haamud, Somalia's police commander, told reporters in the capital, where pedestrians walked in quiet streets.

Presidential candidates have promised to improve security and the economy. Till now, a construction boom in the bombed out capital has yet to spread far across the rest of the nation, where a severe drought is threatening a new food crisis.

The airport in Mogadishu, secured by African peacekeeping force AMISOM and home to U.N. offices and foreign embassies, is the safest site in the capital.

Al Shabaab, which once rule most of Somalia and wants to impose its strict interpretation of Islam on the nation, regularly launches attacks in Mogadishu but its area of control has been increasingly reduced to smaller pockets of countryside.

"All those involved in the election, directly or indirectly are apostates," Al Shabaab spokesman Sheikh Ali Mohamud Rage told Reuters, saying the group would take "tough measures" against those involved and accused foreigners of guiding the vote.

The government of the aid-dependent nation and its Western backers dropped an initial plan to give each adult a vote because of the challenge of securing national polling stations.

Instead, about 14,000 clan elders and regional figures chose the 275 members of the lower house of parliament and 54 members of senate. Those lawmakers will pick the president on Wednesday.

The system is a modest improvement on 2012, when just 135 elders picked parliament, which chose the president.

Experts said President Mohamud, who has had to fend off accusations by Western donors of corruption in his government, can rely on the loyalty of about a third of the new lawmakers, giving him an edge but not a guarantee of victory. (Writing by Edmund Blair, editing by Pritha Sarkar)