Eyewitnesses told the BBC that the unrest had been triggered by the arrest of a lawyer who is an outspoken critic of the government.
The lawyer was later said to have been released, but the demonstrations reportedly continued.
Pro-democracy protests have swept through several Arab countries in recent weeks, forcing the leaders of Tunisia and Egypt from power.
There is no independent confirmation of the protests in Benghazi, but eyewitnesses say that at one stage some 2,000 people were involved.
They say stones were thrown at police who are said to have responded with water cannon, tear gas and rubber bullets.