MANILA, Dec 24 (Reuters) - A crude bomb planted outside a popular fast food outlet injured three people in the southern Philippines on Monday, police said.

The explosion in Cotabato City in front of a branch of Jollibee, the country's most popular fast food chain, happened at lunch-time on Christmas Eve, one of the biggest celebrations in the largely Catholic Philippines.

Senior inspector Samson Obatay told reporters they were looking into the possibility of either a terrorist attack or extortion. Two street vendors and a passerby were injured.

Small-scale bombings are common in the volatile southern Philippines, where there are long-running Muslim and communist insurgencies and bandits sometimes use explosives to extort money.

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