Kenyan police have found explosives after a series of raids on three mosques this week in Mombasa and arrested more than 150 people suspected of being supporters of al-Qaeda-affiliated al-Shabab.

Richard Ngatia, a local police chief, said investigators on Wednesday found a grenade and three machetes at Mombasa's Mina mosque, while two 10-litre petrol bombs and a bomb detonator were uncovered at Swafaa mosque.

Police raided two mosques on Monday, seizing weapons and anti-government literature that they said was evidence of militant activity.

Police had arrested 376 people during the raids, which started on Sunday, but 91 were subsequently released for lack of evidence. Prosecutors said 158 would be charged with being members of al-Shabaab.

Police said they were still considering what to do with the other detainees.

"We have recovered a hand grenade, some petrol bombs, we recovered a bomb detonator, we recovered a booster, we also recovered paraphernalia associated with training in jihadism," Ngatia said.

Kenyan Interior Minister Joseph Ole Lenku said in a statement late on Tuesday that "houses of worship cannot and shall not be taken over by criminals or used for criminal activities, including terrorism".

He also said an al-Shabab flag had been found in one of the mosques. 

Thirteen of those arrested on Monday have been charged with possession of explosive materials and pleaded not guilty. Others held in the mass arrests were still being questioned.

Kenya has boosted its crackdown on armed groups blamed for a series of attacks in the capital Nairobi as well as along its Indian Ocean coastline.

Al-Shabab fighters killed at least 67 people in a gun and grenade raid on a Nairobi shopping mall last September, saying it was revenge for attacks on its fighters by Kenyan troops in Somalia.

Insecurity plagues East Africa's biggest economy, and attacks in the past year on the coast and in the capital have prompted Western nations to issue travel warnings, hitting the tourism industry, which is a big source of hard currency.

Source: Al Jazeera and agencies