Police fired tear gas at demonstrators who were burning tyres outside the US consulate in Lahore. According to a Punjab police spokesperson, a number policemen were injured by the mob attacks.
American Raymond Davis was acquitted earlier the day after $2.34 million in "blood money" was paid to the families of the 27 January shooting to secure the man's release, according to news reports, citing a lawyer representing families.
Raymond Davis, 36, shot dead two men in the eastern city of Lahore on 27 January. He said he defended himself during an armed robbery.
Lawyer Raja Irshad said 19 relatives appeared in court Wednesday to accept the money, the report said.
Pakistani English-language newspaper Dawn said Davis was flown out of the country following his acquittal.
In the cities of Rawalpindi and Islamabad, hundreds of angry youths blocked the traffic by bonfires on the roads.
Jamaat-e-Islami and the other religious parties have called a countrywide strikes for Thursday and Friday to protest Davis' release.