The anniversary of 18-year-old Michael Brown's death in Ferguson, Missouri, began with a solemn march in his honor but ended with a rowdy protest that was interrupted by gunfire.

Shots rang out late on Sunday as several hundred people gathered on West Florissant Street.

St Louis County Police Chief Jon Belmar said a man who opened fire on police is in a "critical, unstable" condition after being struck when the officers returned fire.

Mr Belmar said plainclothes officers had been tracking the man, who they believed was armed, during the protest.

He said the man approached the officers, who were in an unmarked police car and opened fire. The officers returned fire and then gave chase.

When the man again fired, all four officers fired back.

The injured man was taken to a hospital where he underwent surgery.

Before the shooting, protesters were blocking traffic and confronting police.

For the first time in three consecutive nights of protests, some officers were dressed in riot gear, including bullet-proof vests and helmets with shields.

Organisers of some of the weekend activities have pledged a day of civil disobedience today, but have not yet offered specific details.

Several events earlier had marked the anniversary of the killing that cast greater scrutiny on how police interact with black communities. Brown's father, Michael Brown Sr, led a march through the town after a crowd of hundreds observed four and a half minutes of silence.

The group began their silence at 12:02pm, the time Brown was killed, for a length of time that symbolised the four and a half hours that his body lay in the street after he was killed. Two doves were released at the end.

Mr Brown then held hands with others to lead the march, which started at the site where his son, who was black and unarmed, was fatally shot by Ferguson officer Darren Wilson on August 9, 2014.

A grand jury and the US Department of Justice declined to prosecute Wilson, who resigned in November, but the shooting touched off a national "Black Lives Matter" movement.

Pausing along the route at a permanent memorial for his son, Mr Brown said, "Miss you."

He had thanked supporters before the march for not allowing what happened to his son to be "swept under the carpet".

Later, a few hundred people turned out at Greater St Mark Family Church for a service to remember Brown, with his father joining other relatives sitting behind the pulpit.

The two-hour commemoration, featuring a mime dance and a rap-infused version of "Lean on Me" peppered between reflections about Brown, thinned as it wore on. Roughly 50 still remained by the time Mr Brown thanked attendees, saying: "This movement is going to be a good movement."

The anniversary has sparked days of renewed protests, though until Sunday they had been peaceful and without any arrests.