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.

A three-month "state of emergency" that was declared by King Hamad bin Isa Al Khalifa on March 15, is due to be lifted on June 1.

At the height of the protests, up to 200,000 people rallied against the government. The crackdown was an attempt to end the protests that demanded the end of the despotic rule of the Khalifah royal family.

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 [guards] to keep me in the rest-room," she said during the secretly filmed interview.

"He [the officer] hit and banged me against the wall to scream. Since we did not cry out or scream, we were beaten more and more, stronger and stronger.

"Beating was severe, but being afraid of what comes next, we were senseless to the pain."

Bahrain's government has not responded to Al Jazeera's request for comment.

According to the mainly Shia opposition Al Wefaq party, police have raided up to 15 mainly girls schools, detaining, beating and threatening to rape girls as young as 12.

A Bahrain human rights group says at least 70 teachers have also been detained. Meanwhile the media clampdown continues.