Swastikas and racist graffiti have been painted on the walls and windows of a mosque.
The mosque is still under construction and the vandals broke in and used paint taken from builders' cabins on the site.
Officers believe it happened between 2am and 4.30am when the break-in, in Jinnah Road, Redditch, West Midlands, was discovered by a targeted police patrol.
Entry to the site was forced via a gate and the intruders then broke into the main building. It is not known whether anything has been stolen.
Police have informed mosque representatives and are guarding the site and carrying out local reassurance patrols. Forensic examinations are going on.
North Worcestershire police commander Superintendent Kevin Purcell said: "For as long as I can remember, the relationship between the Muslim community in Redditch, the police and the wider community would best be described as excellent.
"Due to incidents happening nationally, targeted patrols have been put in place and these will now be further increased as we will not tolerate mindless attacks of this nature.
"I will be arranging meetings with the chairs of the mosques, local civic leaders and the independent advisory group to reassure them of our commitment and determination to do everything possible to prevent any further such incidents and track down those responsible for this attack."
The incident comes just days after detectives in Walsall launched a major hate-crime investigation when a small home-made bomb exploded near a mosque.
Around 150 people were evacuated from their homes in the Caldmore area of Walsall on Saturday night while bomb disposal experts made the device safe.
West Midlands Police said a loud bang heard by residents late on Friday "appeared to be consistent" with the device exploding. No one was injured and it caused minimal damage.
The remains of the device were found in an alleyway adjoining the Aisha Mosque and Islamic Centre in Rutter Street on Saturday by a local man who took them home and showed them to his wife.
The mosque's imam also took them home later before anyone realised their significance.
Both incidents happened in the wake of a number of similar instances at mosques following the murder of Drummer Lee Rigby in Woolwich, south east London.
Last week two men were charged in relation to an alleged arson at a mosque in Gloucester, and an Islamic cultural centre in Grimsby was hit by petrol bombs last month.
Mosques in Braintree, Essex, and Gillingham, Kent, have also been targeted.
West Mercia Police have appealed for anyone with any information relating to the Redditch incident to call officers at Redditch police station on the non-emergency number 101, quoting incident reference 20-S-260613, or independent charity Crimestoppers anonymously on 0800 555 111.