Four mosques in Birmingham have been targeted in a string of violent attacks overnight and into the morning.
Reports of a man smashing windows with a sledgehammer on Birchfield Road were received at 02:30 GMT, police said.
Officers were then alerted to a similar attack in Erdington about 45 minutes later, with more in Aston and Perry Barr reported. Another on Albert Road was struck at 10:00 GMT.
West Midlands Police said it was yet to establish a motive.
Officers from its Counter Terrorism Unit were investigating, the force said.
Yousef Zaman, chairman of Masjid Faizul Islam mosque in Aston, said: "My initial reaction was shock that this had happened.
"There's a fear factor now in that adults are saying they're going to keep their children away from the mosque today because they're worried that it's not safe.
"But we're not going to stop worship, we're going to carry on as normal, we won't let them win, we will defy them."
He said a summit was planned to discuss security around the mosques.
A spokesperson for Witton Islamic Centre on Witton Road, also in Aston, said CCTV captures a man smashing windows at about 01:30.
"The whole of the front windows, about six, were smashed," he said.
"Because of the force he used it's gone through the windows and into the mosque itself".
Councillor Majid Mahmood tweeted a video of the clean-up taking place at the centre.
In a press conference held outside the Witton Islamic Centre, West Midlands Police and Crime Commissioner David Jamieson said: "What we have today is broken windows, but what people are doing who are breaking those windows are trying to break [is] our spirit, and break the cohesion that exists between all the people of the varied faiths and diverse community that we have here in the West Midlands.
"What I'm here today to say is that those people are not going to succeed and people who carry out this type of criminality will be hunted down relentlessly."
Officers said they will talk to mosques about extra security at Friday Prayers.
Chief Constable of West Midlands Police Dave Thompson said: "At the moment we don't know the motive for last night's attacks.
"What I can say is that the force and the Counter Terrorism Unit are working side-by-side to find whoever is responsible."
Councillor John Cotton, cabinet member for social inclusion, community safety and equalities with Birmingham City Council, said he was "appalled" by the violence and was working with police to find those responsible.
He tweeted: "These thugs do not speak for Birmingham and will not divide us."
A statement from the Birmingham Council of Mosques said: "We were deeply horrified to hear a number of mosques were vandalised during the early hours of this morning.
"Birmingham's mosques are a place of worship, serenity and a source of peace and tranquillity. We are appalled by such acts of hate/terror."
Home Secretary Sajid Javid called the vandalism "deeply concerning and distressing", while MP for Birmingham Ladywood Shabana Mahmood said the attacks were "truly terrible".
In the Commons, the Labour MP for Birmingham Erdington, Jack Dromey, spoke out in support of the city's Muslim community.
While the leader of the Commons, Andrea Leadsom, was scathing of those responsible for the attacks.
She said: "Our hearts go out to those who are affected by these attacks in mosques in Birmingham last night, it's absolutely unacceptable to see any form of religious or racial, or any form of prejudice whatsoever in our free and open society."
Mr Thompson added: "Since the tragic events in Christchurch, New Zealand, officers and staff from West Midlands Police have been working closely with our faith partners across the region to offer reassurance and support at mosques, churches and places of prayer."
