The initiative, launched last year by the Muslim Council of Britain (MCB), attracted more than double participating mosques this year and is aimed at countering what the MCB describes as "hostile headlines" on the faith.
Mosques in London, Birmingham, Manchester, Leeds, Glasgow, Cardiff, Belfast, Plymouth and Canterburyare among those participating in the #VisitMyMosque programme.
"The day seeks to provide a platform for Muslims to reach out to fellow Britons and explain their faith and community beyond the hostile headlines," the MCB said in a statement.
During the open day, introductions to Islam and explanations on Muslim community activities were offered, as well as opportunities to see prayers and guided tours.
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The latest census, from 2011, showed there were 2.7 million Muslims in the UK -- about 4.5 per cent of the population.
The 'Visit My Mosque' event comes amid rising anti-Muslim sentiments in Britain and all across Europe.