Protesters have rushed City Hall in St Louis to hold a mock trial of a police officer who shot dead an unarmed black man.

It has led police to lock down the building and call in more than a hundred additional officers.

The demonstrators are angry at a grand jury's decision not to charge a white police officer for the killing of unarmed black teenager Michael Brown.

Protesters shouted "Shame, shame" while running into the building and at least three people were arrested as the site was locked down by officers.

Those activists who got inside were part of a group of about 200 protesters who marched and held a mock trial of Ferguson Police Officer Darren Wilson who shot dead Mr Brown on 9 August.

It followed a second night of unrest in Ferguson following Monday's decision by a 12-member grand jury not to indict Officer Wilson.

Additional National Guard troops deployed to the St Louis suburb helped make protests on Tuesday night much calmer than on Monday when looters ransacked local businesses and set police cars and a dozen buildings ablaze.