David Cameron apologised for not wearing traditional Indian dress as he arrived with his wife wearing a navy suit.
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But his forehead was marked with a tilak – a red dot worn by Hindu worshippers on religious festivals.
The Prime Minister and Samantha removed their shoes as a mark of respect before entering the Swaminarayan Mandir temple in Neasden.
The place of worship is the largest of its kind outside India and is visited by British Hindus, Jains and Sikhs.
Mr Cameron spoke to thousands of worshippers gathered in the temple's main hall. He told them: "I think of the values that you are celebrating today – family, community.
Those are the values that we in this country need more of. It is great that more British Indians are contributing to British politics. We want many more British Indians in our Parliament, Commons and Lords.”
The British leader told how he will soon make his third official visit to India.
"This is a vitally important friendship for the United Kingdom," he said.
"Yes we have a shared language, we have cultural ties, we have a shared past, but what really matters is how we can work together now and in the future.
"We want to be the partner of choice for India to grow, to do business, and to make that case I want to have you right behind me."