The Prince of Wales and Duchess of Cornwall have visited the spectacular Sheikh Zaved Grand Mosque to promote religious tolerance.
Charles and Camilla first visited the mosque, in Abu Dhabi, in 2007 when it was still being built.
They returned as part of their tour to the Middle East on behalf of the British Government, having left Oman earlier on Sunday.
Charles was dressed in a linen suit and striped tie, while Camilla wore a blue headscarf, long jacket and trousers.
Visitors to the mosque must remove their footwear, and Charles walked round in black socks while his wife went barefoot.
The couple toured the mosque, which has stunning chandeliers and a wall featuring the 99 names of God in traditional calligraphy, before learning about its architecture.
The carpet in the main hall of the mosque, which took 11 years to build, is considered to be one of the largest in the world.
Following the tour, Charles and Camilla attended a reception for guests of different faiths and nationalities.
Bishop Paul Hinder, of the Roman Catholic Church, spoke with Charles at the mosque.
"For me, the visit is about recognition," he said.
"The prince was able to speak to us and see the mutual tolerance which is reality in this country.
"It doesn't happen all over the world that we can meet in such a place in such a formation.
"Living and seeing this reality has an impact."
The mosque was established in 2008 and sits at the entrance to Abu Dhabi City Island.
It aims to work with research centres and religious, educational and cultural institutions within the United Arab Emirates and across the world.
Charles and Camilla were shown around the mosque by Sheikha Lubna Al Qasimi, who was appointed as the new minister of state for tolerance in February.
The UAE is home to more than 200 nationalities, a large majority of whom are Commonwealth citizens.
Kim Debenham, deputy head of mission at the Australian embassy, said: "It was my first opportunity to meet anyone from the British royal family so it was quite special.
"Prince Charles said he had visited Abu Dhabi before but the last time he was at the mosque it was a construction site.
"We spoke about the different commonwealth communities here in Abu Dhabi and that they are growing all the time."
Before travelling to Abu Dhabi, Charles and Camilla carried out engagements in Muscat, Oman.
Charles joined a meeting of young leaders who had a connection to the UK before speaking to three people who recreated the journey of British explorer Bertram Thomas.
He crossed the Empty Quarter - one of the hottest and most inhospitable places on Earth - on foot and by camel in 1931.
Mark Evans, Mohammed Al-Zadjali and Amour Al-Wadabi set off on the journey in March and took 49 days to complete it.
Camilla visited the Dar Al Atta Bookshop, where she joined local school children participating in reading activities.
She then saw the mobile library - the country's first - outside.
The Duchess opened Oman's first children's public library and spoke to three young children who won a local writing competition.
Charles and Camilla left Oman and flew to Abu Dhabi, where they were welcomed by the minister of foreign affairs and international cooperation and HM ambassador to the United Arab Emirates.