Secret filming conducted by Al Jazeera has revealed shocking evidence of the brutal crackdown against pro-democracy protesters in the Gulf state of Bahrain.

An undercover investigation conducted by Al Jazeera's correspondent, Charles Stratford, has unearthed evidence that Bahraini police carried out periodic raids on girls' schools since the unrest began.

The government of Bahrain deployed security forces onto the streets on March 14 in an attempt to quell more than four weeks of protests.

In an interview, "Heba", a 16-year-old schoolgirl, alleges she, along with three of her school friends, were taken away by the police from their school and subjected to severe beatings while in custody for three consecutive days.

"He hit me on the head, I started bleeding. I fell down, he told them [the guards] to keep me in the rest-room," she said during the secretly filmed interview.

Al Jazeera's Charles Stratford reports from Bahrain.