UPDATE 1-Supporters of Iran election rivals clash - witness

* Second night of sporadic clashes in capital

* Incident takes place as rivals face off in debate

(Adds quotes from debate, background)

TEHRAN, June 6 (Reuters) - Supporters and opponents of Iranian President Mahmoud Ahmadinejad clashed in a Tehran square on Saturday evening and some cars were set on fire, a witness said, in a sign of rising tension ahead of the June 12 election.

The incident took place as rival supporters followed a televised debate between Ahmadinejad and pro-reform challenger Mehdi Karoubi on big screens at Sarv square in northwestern Tehran, the witness told Reuters.

It was the second night of sporadic unrest in the capital, after thousands of supporters of Ahmadinejad and another of his moderate challengers in the presidential election scuffled elsewhere in the capital on Friday evening.

Mainly young supporters of the election hopefuls have poured into the streets of Tehran during evenings as the election approaches, shouting, honking car horns and waving pictures of their candidates. At times, traffic has ground to a standstill.

With more than 60 percent of Iranians under the age of 30, their votes will be crucial.