Government forces and opposition fighters have exchanged heavy gunfire in Somalia's capital, Mogadishu, as a row over delays to elections has escalated.

President Hassan Sheikh Mohamud's term in office ended on 15 May but was extended by a year. The opposition said this was unconstitutional and called for protests on Thursday.

Gunfire was reported in several neighbourhoods of the capital and persisted throughout the night, according to accounts by residents.

Police said they were conducting a "large-scale security operation" against "heavily armed militias who launched mortar attacks" in some areas.

Following the expiry of the president's term, the federal government and the opposition started talks but did not reach an agreement.

President Mohamud has been attempting to move Somalia towards democratic elections, replacing a system in which clan elders choose MPs, who in turn select a president.

Somalia last held a one-person, one-vote election in 1969 and has been ravaged by civil war for more than 30 years.

Former Prime Minister Hassan Ali Khaire said he had been attacked by government forces while he and other leaders were preparing for Thursday's "peaceful" demonstrations.

"The responsibility for any casualties or damage resulting from this incident lies with the president whose term has expired," he said on X.

"This attack is a grave assault on the constitutional rights of Somali citizens and a deliberate attempt to suppress peaceful assembly," he added.

The number of casualties is not yet known and the president has not commented.

In statement on X, former President Sharif Sheikh Ahmed said the violence would not halt the planned demonstrations.

"If the President and his soldiers think that we are afraid or that we will flee, we are not going to run away," he said.

The US embassy in Mogadishu has described the violence as "reckless", adding that leader on all sides "have a responsibility to preserve stability and resolve differences through peaceful means".