BANGKOK, Thailand - A grenade exploded in the compound of a Thai state-run broadcaster on Tuesday, damaging vehicles but causing no injuries, the fourth mysterious blast in five weeks during a state of emergency in Bangkok.

Police suspect the grenade was fired by a shoulder-mounted M79 launcher from a highway overlooking the National Broadcasting Services of Thailand (NBT) and exploded after hitting a tree inside its compound, sending shrapnel into the parking lot.

"Three vehicles were slightly damaged by sharpnel from the explosion," said Police Colonel Paisan Wongwatcharamongkol.

The NBT has faced criticism from "red shirt" anti-government demonstrators and independent media observers who accuse the channel of biased reporting. It is near one of Bangkok's largest military bases on an expressway leading into the capital.

Authorities did not blame anyone for the attack. An opposition leader denied involvement in the incident.

"I don't know who did it. We criticise the channel because it is irresponsible in its reporting but that's different from attacking it. We don't use such tactics," protest leader and parliamentarian Jatuporn Prompan said.

It was the latest in a series of small grenade or bomb attacks across Bangkok since violent clashes between troops red shirt protesters in April and May, in which 91 people were killed and up to 2,000 wounded.

On Aug. 26, a grenade fired into the headquarters of Thai duty-free giant, King Power Group, wounded a security guard. That occurred on the same street where a bomb exploded a few weeks earlier, seriously wounding a garbage collector.

On July 25, a bomb hidden at a bus stop outside a supermarket killed one man and wounded nine.

The emergency decree, imposed on April 8 after protesters stormed parliament, gives security agencies broad powers to impose curfews, detain suspects without charge, censor media and ban political gatherings of more than five people.

Prime Minister Abhisit Vejjajiva has not identified when emergency rule will be lifted in Bangkok.

Broadcaster NBT was targeted for criticism in 2007 by "yellow shirt" protesters when allies of former prime minister Thaksin Shinawatra were in power.

Additional reporting by Panarat Thepgumpanat; Editing by Jason Szep and Sugita Katyal