MUMBAI — Indians are paying their tributes to popular singer KK, who died unexpectedly on Tuesday night, hours after performing at a concert.

The 53-year-old, whose real name was Krishnakumar Kunnath, sang hundreds of songs over his career, which began in the 1990s.

He collapsed at his hotel in Kolkata city and was rushed to a hospital, where doctors pronounced him dead.

The cause of death hasn't been confirmed yet.

On Wednesday morning, 'Not KK' trended on Indian Twitter as fans and colleagues registered their shock and disbelief at the news.

Prime Minister Narendra Modi led the tributes, tweeting his grief at the singer's "untimely demise".

"His songs reflected a wide range of emotions and struck a chord with people of all age groups. We will always remember him through his songs," Mr Modi said.

KK's death, two days after the murder of Punjabi rapper Sidhu Moose Wala, came as a double blow to music lovers in India this week.

Born in Delhi in 1968, KK began his career singing advertising jingles - he sang more than 3,500 of them before getting his big break in film music - and went on to perform in several Indian languages, including Tamil, Telugu and Kannada.

But it was his work in the Hindi music industry that made him the voice of a generation.

He made his Bollywood debut as one of the four singers of the powerful Chod Aaye Hum Voh Galiyan (We've Left Behind Those Streets) - on the comradeship between four young men on a rebellious journey - in the 1996 movie Maachis, which was set against the backdrop of the insurgency in Punjab.

Friendship and the bittersweet weight of nostalgia were recurring themes in KK's songs - this was perhaps best captured in the soulful Pal (Moment) and Yaaron (Friends). The lyrics, along with the velvety youthfulness of his voice, struck a chord with millennials coming of age in the late 1990s and early 2000s, who would hum along to it at parties and perform it at college festivals.

KK was also the voice behind several iconic romantic songs many of which were produced by music composer Pritam and duo Vishal-Shekhar. His biggest hits included Tadap Tadap Ke, Khuda Jaaney, Tu Hi Meri Shab Hai, Kya Mujhe Pyaar Hai and Dus Bahane.

His ability to evoke anger, passion and yearning - that had been largely excluded from the vocabulary of pop songs - solidified his position in the music industry, spurring a new genre of rock-style music that featured slick sounds of the electric guitar and drums.

Many of his colleagues from the Indian music industry remembered their association with him.

Acclaimed singer Shreya Ghoshal tweeted, "This is too hard to accept! Heart is shattered in pieces."

Musician Shankar Mahadevan paid his respects with a line from KK's song Pal: "Hum rahe ya na rahen yaad aayenge ye pal (We'll remember these moments whether we're here or not.)"

Actor Emraan Hashmi, whose most popular songs were sung by KK, said the singer would "live eternally" through his music.

KK's last Instagram post showed him standing in front of a packed crowd at his final concert at Kolkata.

The post was flooded with comments from tearful fans after news of his death broke.

KK was in Kolkata to perform at two college festivals, The Times of India reported.

"He was brilliant at the show and was the showstopper," the newspaper reported, quoting a spectator at the concert.

KK is survived by his wife and two children. — BBC