FERGUSON (Reuters) - Riot gear-clad police officers arrested at least 17 people on Sunday after they refused orders to disperse from a spontaneous sit-in outside a convenience store in St Louis during a weekend of otherwise peaceful protests against police violence.

Thousands of people staged protest marches, vigils and other demonstrations in the St Louis area on the weekend, calling for the arrest of a white police officer who shot dead an unarmed black teenager in August.

Another fatal shooting of an African-American teenager by an off-duty cop last Wednesday has further inflamed tensions. Sunday's arrests were in the same neighborhood where Wednesday's shooting occurred.

St Louis police spokesman Schron Jackson said 17 people had been arrested for unlawful assembly early on Sunday at the parking lot of the QuikTrip convenience store. They were released later on Sunday. She did not say if they faced charges.

Mr Mervyn Marcano, who is handling media relations for a group that provides jail support for protesters, said at least 19 people had been arrested.

St Louis police chief Sam Dotson tweeted during the protest that protesters were throwing rocks at the police. But the protesters tweeted that they had not thrown anything.

Witnesses transmitting live video from the small overnight protest in Shaw, a neighborhood of St Louis, showed a few dozen people sitting on the ground outside the convenience store and said some people were hit with pepper spray and what appeared to have been tear gas.

The police said they used pepper spray and not tear gas.

Last Wednesday, an off-duty white officer working for a security firm shot dead 18-year-old Vonderrit Myers Jr. in the Shaw neighborhood in what the police described as a firefight.

There were no reports of arrests at protest rallies on Saturday that drew thousands of people. The police have largely adopted a non-confrontational stance and protest organizers work to maintain order and a non-violent approach.

The mother of Michael Brown, the 18-year-old shot dead on Aug 9 by a white officer in Ferguson, Missouri, walked at the front of a rally on Saturday evening in the suburb of Ferguson.

Ms Lesley McSpadden, who has criticized Ferguson police, has only rarely participated in protests. She eventually left the group, which grew to over 1,000 people and moved on to Ferguson police headquarters.

Brown's death triggered a national uproar in August over police accountability and protesters have called for the arrest and prosecution of the officer Darren Wilson. A grand jury is now considering the case.

At the culmination of the Ferguson march on Saturday night, protesters shouted insults at a line of police in helmets and shields, with some demonstrators wearing bandanas or scarves over their faces. But there were no arrests or violence, a police spokesman said.

Earlier on Saturday, thousands of protesters in downtown St Louis marched and rallied at a plaza, where organizers included Hands Up United, an activist group that emerged after Brown's death.

Union members, gay rights activists and people from the Occupy movement joined in. Civil rights organizations and protest groups had invited people from across the United States to join vigils and other weekend events in the St Louis area.

"This isn't going to stop until there is change with police and black youth," said Mr Tory Russell, a founder of Hands Up United.