Heavy gunfire has broken out in Somalia's capital as opposition leaders defied a ban on public gatherings to protest about delayed elections.

The president's term expired last week, without the selection of a successor, throwing Somalia into a crisis.

Overnight, the government said militiamen attacked army positions in Mogadishu, close to the presidential palace.

Most roads in the city have been closed and special forces deployed.

President Mohamed Abdullahi Mohamed, also known as "Farmajo", is expected to hold a meeting with Somalia's five regional presidents as part of efforts to end the deadlock.

International flights to Mogadishu's main airport have been suspended following heavy gunfire and shelling by the perimeter wall, reports the BBC's Bella Sheegow from the city.

The opposition march was proceeding along the main airport road when the gunfire broke out.

Reports say security forces shot in the air to disperse the protest.

"Many forces heavily attacked us, I am now on my chest in an alley," protester Farah Omar told the Reuters agency by phone.

The UN has said it is "deeply concerned" by the clashes between the opposition and security forces overnight and on Friday morning.

It has called for "calm and restraint by all parties involved" and urged "that open lines of communication be maintained to help reduce tensions".

Analysts say there is concern that the political wrangling could split the security forces along clan lines at a time when they are fighting an Islamist insurgency.