Tomorrowland brings together 400,000 people over two weekends in July. (AFP)

A man attending Belgium's Tomorrowland festival, one of the world's biggest electronic music events, collapsed and was later declared dead in hospital, organisers and medical sources said Saturday.

The 27-year-old from India fell unconscious while at the festival on Friday, Antwerp prosecutors told VTM news.

He was transported from the site in the town of Boom to Antwerp University Hospital where he could not be revived, a hospital spokeswoman told AFP on Saturday.

An autopsy was to be carried out on Monday.

In a statement to Belga news agency, Tomorrowland organisers announced "with sadness the death of one of its guests".

Spokeswoman Debby Wilmsen told VTM the cause of death was not yet known.

She added that the festival had zero tolerance with regard to drugs, a common feature of many big European electro gatherings.

Tomorrowland brings together 400,000 people over two weekends in July to see many of the biggest names in electronic dance music.

The spokeswoman for Antwerp University Hospital, Evita Bonne, said the Indian was among four people from the festival admitted Friday for intensive care. One remained in the hospital on Saturday.

Last year, a 26-year-old woman, also from India, became unwell at the festival and died one day later in the hospital. A Czech woman also died.

Both victims had traces of of the drug ecstasy in their blood. But according to authorities their causes of death were due to a combination of factors, media reported.

In 2012, a 26-year-old British woman succumbed to a drug overdose.

This year's festival includes more than 130 performers including Eric Prydz, former NBA star Shaquille O'Neal, fashion designer Virgil Abloh, Steve Aoki, DJ Tiesto and Dave Clarke.