* Unrest follows announcement that Widodo won April 17 vote

* Police fire tear gas, rubber bullets, water cannon at protesters

* "Mobs are paid and bent on causing chaos," police spokesman says

* Prabowo to mount legal challenge to election result on Thursday

By Kanupriya Kapoor and Fanny Potkin

JAKARTA, May 23 (Reuters) - A clash between Indonesian security forces and protesters angry over the result of last month's presidential election raged for a second night on Wednesday, turning central Jakarta into a battlefield of tear gas, rubber bullets, rocks and firecrackers.

Protesters tore up slabs of pavement, destroyed street signs and set fire to food stalls and security posts but their numbers thinned as the night wore on. Indonesian TV showed dozens of riot police trying to sleep on debris-strewn streets.

The capital's governor, Anies Baswedan, said six people were killed in the first night of rioting, which was concentrated in the sprawling textile market neighbourhood of Tanah Abang. State news agency Antara reported that three hospitals had so far treated more than 350 people for injuries.

The unrest followed an announcement on Tuesday by the General Election Commission confirming that President Joko Widodo had beaten his challenger, former general Prabowo Subianto, in the April 17 poll.

Widodo won more than 85 million of 154 million votes cast but Prabowo alleged "massive cheating and irregularities" and refused to concede defeat. The election supervisory agency has said there was no evidence of systematic cheating, and independent observers have said the poll was free and fair.

'PAID MOBS'

A crowd of protesters swelled outside the supervisory agency's headquarters on Wednesday, some carrying wooden poles and some with toothpaste smeared around their eyes to mitigate the effects of tear gas.

Many left peacefully, but as night fell others hurled firecrackers and other objects at officers and set blazes as they tried to breach barbed wire separating them from police. Police responded with tear gas, rubber bullets and water cannon.

Medics were seen treating dozens of protesters affected by tear gas and using oxygen to revive some who had passed out.

Many of the protesters appeared to have come from outside Jakarta and police found envelopes containing money on some of the people they searched, national police spokesman Muhamad Iqbal told a news conference on Wednesday.

"This is not a spontaneous incident, this is something by design. There are indications that the mobs are paid and bent on causing chaos," he said.

Vice President Jusuf Kalla called on Prabowo and his running mate, Sandiaga Uno, to rein in their supporters.