Police watch protesters on January 9, 2012 in Lagos during a demonstration. File picture: AP.

ABUJA - Police opened fire at Shiite Muslim protesters demanding the release of their leader in the Nigerian capital Abuja on Tuesday in a second straight day of violence, a Reuters witness said.

Troops had fired on a march on Monday by members of the Islamic Movement of Nigeria (IMN). The group said the soldiers killed at least 20 protesters but the military said only three people died and four soldiers were injured.

On Tuesday, police attempted to stop hundreds of IMN members as they headed towards the centre of Abuja, where most government buildings and the presidency are located.

They were demanding the release of their leader Ibrahim Zakzaky, who has been jailed since December 2015 when security forces killed hundreds in a crackdown on the group. 

Zakzaky has been held for more than two years without charge despite a court ruling that he be released.

When marchers refused to turn back, police fired tear gas on the crowd. The protesters responded by hurling rocks, and the police opened fire, according to the Reuters witness.

As the violence escalated, at least one IMN member was wounded by the gunfire and the protesters set ablaze a police car, the witness said.

A police spokesman did not respond to calls and messages seeking comment.

IMN protests have frequently met with force. In April, police fired bullets and tear gas during days of protests by IMN, wounding at least four.

The repression of IMN, which is estimated to have 3 million followers, and the detention of its leader have drawn accusations from international rights watchdogs that President Muhammadu Buhari's government is abusing human rights.

The crackdown has raised concern that IMN could become radicalised, in much the same way that the Sunni Muslim militant group Boko Haram turned into a violent insurgency in 2009 after police killed its leader.