Kenyan security forces on Wednesday carried out fresh raids on a mosque in the port city of Mombasa searching for weapons and supporters of Somalia's Shebab militants, police said.
Grenades, ammunition and petrol bombs were seized in the raid on the Swafaa mosque, the third to be searched since Monday in the tense city, officers said.
"We conducted a security operation," local police chief Richard Ngatia said, adding that "weapons were seized" but that the operation was ongoing.
Dozens of people were arrested, adding to more than 250 arrested on Monday.
Security forces cordoned off streets around the mosque and soldiers patrolled the Kisuani district of the city, east Africa's main port, an AFP reporter said.
It comes just two days after police raided and closed the Musa and Sakina mosques in the same city in a search for weapons and radical supporters of the Al-Qaeda affiliated Shebab.
Police seized hand grenades and a pistol, and arrested more than 250 people. Black Islamist flags, such as those flown by the Shebab, were also found.
"I want to make it clear that houses of worship cannot and shall not be taken over by criminals or used for criminal activities, including terrorism," Kenyan Interior Minister Joseph Ole Lenku said in a statement late Tuesday.
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.
"I want to assure Kenyans that this process will separate those with criminal intent from the innocent Kenyans who will be released immediately," Lenku added.
Four people were stabbed to death in apparent revenge attacks on Monday, with gangs taking to the streets beating some and knifing others.
The raids have raised tensions in a city already hit by a string of bombings and shootings.
Hussein Khalid, from the Mombasa-based civil society group HAKI Africa, argued that "force will only act to heighten tension in what is already a volatile situation".
Western nations have warned their nationals to avoid all but essential travel to Mombasa, a key transport hub as well as an important tourist centre for the country's Indian Ocean coastline.
Kenya has suffered a series of attacks since invading Somalia in 2011 to attack the Shebab, later joining an African Union force battling the Islamists.
The Shebab carried out the September 2013 attack on Nairobi's Westgate shopping mall, killing at least 67 people as a warning to Kenya to pull its troops out of southern Somalia.
Several Islamic preachers have been shot dead in Mombasa in recent years in alleged extra-judicial killings by security forces and power struggles between rival Muslim factions. Churches have also been attacked.