This is since the murder of 50 people by a right-wing extremist at mosques in Christchurch, New Zealand.
West Midlands Police said detectives and counter-terrorism officers are investigating after windows were smashed at the mosques in Birmingham.
Officers responded after reports of a man seen smashing windows with a sledgehammer at one mosque, the police said.
What is going on in our country? We are listing into a dark place, tweeted TellMAMA, a group monitoring anti-Muslim incidents.
The group said it had seen an increase in reports of Islamophobia since the Christchurch murders, citing seven other incidents across Britain.
Attacks on five mosques in Birmingham show, once again, that Muslims are having to bear the brunt of the absolute hatred and bigotry of others, Iman Abou Atta, TellMAMAs director, said in a statement.
People cannot stand by now and say that it does not affect them, Atta said. It affects our society, our communities and our country, and we must stand up against it.
The police said neighbourhood officers were working closely with mosques across the region on Thursday.
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, West Midlands Police Chief Constable Dave Thompson said in a statement.
At the moment we don't know the motive for last nights attacks, Thompson added. At difficult times like this, it is incredibly important that everyone unites against those who seek to create discord, uncertainty and fear in our communities, he said.
Shabana Mahmood, a member of parliament for Birmingham, said the reported attacks were truly terrible. I would urge all residents to remain calm and call the police with any info you may have, Mahmood tweeted.
After the Christchurch attacks, Harun Khan, head of the Muslim Council of Britain, urged the government to redouble its efforts to ensure mosques are protected.
Khan also urged fellow Muslims to resist the temptation to roll up the banners in fear, as this attack was designed to do.
Londons Labour mayor Sadiq Khan said police in the British capital would increase patrols near mosques.
Writing in Tuesdays Guardian newspaper, more than 350 imams, scholars and other Muslim leaders appealed to Western governments and media to do more to confront the systemic and institutionalised Islamophobia underlying the Christchurch attack.