Supporters of the opposition al-Wefaq party in Bahrain last week staged a demonstration after a formal protest permit was denied by the Ministry of Interior.
Human Rights Watch said riot police fired flash grenades and tear gas at a crowd of roughly two dozen who appeared to be protesting peacefully.
Joe Stork, deputy director of the Middle East program at Human Rights Watch, said the apparent excessive use of force against demonstrators was a serious matter.
The Interior Ministry issued a statement to the organization saying it would investigate the matter. Al-Wefaq said five protesters were arrested for demonstrating without a permit.
"An official investigation into this incident is certainly warranted, leading to disciplinary action, and criminal prosecution of officers shown to have endangered lives with reckless use of crowd control devices," Stork said in a statement from Beirut.
The ruling monarchy was criticized by large parts of the international community for its crackdown on dissidents during an uprising last year. An appeals court in Bahrain, however, recently reduced sentences or acquitted some those accused of taking part in anti-government protests.
At least 50 people were killed during last year's uprising.