UPDATE 10-Hong Kong mourning for student spirals into street violence

* First death of student in months of unrest

* Graduation day vigils deteriorate into clashes

* Police spokeswoman in tears as she pledges investigation (Adds tear gas fired in Mong Kok, Tseung Kwan O, paragraphs 1, 4)

By Clare Jim and Jessie Pang

HONG KONG, Nov 8 (Reuters) - Candlelight Hong Kong vigils mourning a student who died on Friday after a high fall during a pro-democracy rally quickly spiralled into street fires, bursts of tear gas and cat-and-mouse clashes between pro-democracy protesters and police.

The centre of violence was on Nathan Road, in the Kowloon district of Mong Kok, one of the most densely populated locations in the world, where activists built barricades and trashed an entrance to the metro station.

Police used a robot to detonate a suspected explosive device on a side street after at least three blasts in the area amid a standoff with petrol-bomb throwing protesters lasting hours.

Police fired tear gas there and in Tseung Kwan O, to the east of the Kowloon peninsula, where the student, Chow Tsz-lok, fell from the third to the second floor of a parking lot in the early hours of Monday.

Chow, 22, who studied at the University of Science and Technology (UST), fell as protesters were being dispersed by police.

He died on Friday - graduation day for many UST students. His death is likely to fuel anger at police, who are under pressure over accusations of excessive force as the former British colony grapples with its worst political crisis in decades.

UST students trashed a campus branch of Starbucks, part of a franchise perceived to be pro-Beijing, and rallies are expected across the territory over the weekend.

"Condemn police brutality," they wrote on the restaurant's glass wall.

Hundreds of students, most in masks and carrying candles, then lined up in silence at UST to lay white flowers in tribute.

Thousands also left flowers at the spot where he fell at the car park, occasionally singing hymns.

In the shopping district of Causeway Bay, hundreds lined the streets in silence, with the eerie hum of the city in the background.

Then the mood changed.

People started shouting abuse at "black police", referring to perceived brutality, and blocked streets in Causeway Bay.

In Mong Kok, dozens of activists barricaded off Nathan Road, which leads to the harbour to the south. They vandalised a closed metro entrance, throwing in bricks and pouring oil through the metal grill, and destroyed a phone booth in a small explosion. There were clashes and fires in the New Territories town of Sha Tin.