Demonstrations, mostly peaceful but with spasms of violence by smaller groups, have flared since unarmed black youth Michael Brown, 18, was shot dead by a white policeman while walking down a street on August 9.
State Highway Patrol Captain Ron Johnson said "our officers came under heavy gunfire" in one area.
He added: "Not a single bullet was fired by officers despite coming under heavy attack." Riot police had confiscated two guns from protesters and what looked like a petrol bomb. Four officers were injured.
Captain Johnson said two people were shot within the crowd, but not by police, and were taken to hospital.
The violence has captured headlines around the world, raising questions about the state of race relations in America.
"This has to stop. I don't want anybody to get hurt. We have to find a way to stop this," said Captain Johnson, an African-American who took over security efforts after the mostly white local force was accused of using excessive force against black people.
An overnight curfew has been imposed and the National Guard has been deployed in the St Louis suburb of 21,000 people to stop looting and fires that have punctuated the protests.
More than 40 FBI agents are canvassing local neighbourhoods and an extra medical examination is to be performed on Mr Brown, who was shot at least six times.
The policeman who shot him, Darren Wilson, 28, is on paid leave, in hiding and under criminal investigation.