It is one of the biggest birthday celebrations in the world: millions of Sikhs will Tuesday mark the 550th birth anniversary of the founder of their religion, the Guru Nanak.

The annual celebration has been given extra significance this year with the opening of the Kartarpur Corridor, a secure, visa-free passage between arch-rivals India and Pakistan that gives Indian Sikhs access to the place where the guru died in 1539, now one of the religion's holiest sites.

Here are some more facts about Sikhs on the anniversary.

Sikhism is a monotheistic religion born in the 15th-century in Punjab - meaning the land of five rivers, a region spanning parts of what is now India and Pakistan.

Five and a half centuries later, Sikhs number up to 30 million worldwide, with a rich diaspora in places such as Canada, the US and Europe - although the vast majority remain in India.

Once rulers of the Punjab, they were considered a martial race by the British colonisers.

Men, in particular, are instantly recognisable by their colourful turbans, used to cover their hair, which they must leave uncut.

That is one of the five articles of their faith they must carry all the time, known as the Five Ks: the Kesh (uncut hair), Kara (a steel bracelet), Kanga (a wooden comb), Kaccha (cotton underwear), and Kirpan (a curved dagger).

Copyright Agence France-Presse, 2019