The unemployed demonstrators gathered outside a mosque in the Makharqa neighbourhood in the centre of the Bahraini capital of Manama on Sunday to demand jobs for the country's jobless.
The Secular Islamic Council, which groups prominent Shia clergymen, said in a statement distributed at the start of the demonstration that "a number of protesters ... were beaten by security forces" in the protest on Friday.
"A number of protesters were injured and taken to hospital for treatment."
A group representing the unemployed protesters said in a separate statement that "dozens of participants in [Friday's] demonstration by the unemployed were injured when police beat them".
Protests are rare in the conservative Gulf monarchies, where citizens are usually banned from demonstrating to press social demands.
Labour unions have been authorised in Bahrain since 2002 as part of attempts to introduce reforms.