BANGKOK - A suspected bomb blast ripped through a bus stop in central Bangkok Sunday, leaving at least nine people injured and reigniting tensions two months after the end of deadly opposition protests.

The explosion came hours after polls closed in a closely watched by-election in the capital pitting a leader of the "Red Shirt" anti-government movement against a member of the elite-backed ruling party.

The blast occurred at a bus stop in front of a Big C Ratchadamri supermarket, in the same central shopping district occupied by the Reds during their mass rally which ended in May, Police Colonel Saravut Jindatham told AFP.

The area was sealed off by police and a bomb squad was seen inspecting the site, according to an AFP photographer.

Saravut he was waiting for forensics to determine the cause of the blast, which Thai media reports had been caused by a bomb hidden in a plastic bag.

The explosion shattered an uneasy calm that has returned to the capital since the military crushed the Red Shirts` two-month-long mass protests in May.

Thai society remains deeply divided following the political violence, in which 90 people died and about 1,900 were injured in a series of street clashes between armed troops and demonstrators, as well as numerous grenade blasts.

About one fifth of the country, including Bangkok, remains under a state of emergency which bans public gatherings of more than five people and give security forces the right to detain suspects for 30 days without charge.

The protests by the Reds -- many of whom back fugitive ex-premier Thaksin Shinawatra -- attracted up to 100,000 people demanding immediate elections, but most of the leaders are now behind bars.

Sunday`s blast came hours after polls closed in a by-election in a suburb of the capital seen by many as a referendum on the authorities` handling of the crisis.

A Red Shirt leader, Kokaew Pikulthong, who is in prison on terrorism charges, appeared to be heading for defeat with 40.93 percent of the votes, according to exit polls.

Ruling Democrat Party candidate Panich Vikitsreth looked set to clinch victory with 52.77 percent, according to exit polls by Rajabhat Suan Dusit University.

Kokaew, a candidate for the opposition Puea Thai (For Thais) party -- who has not been convicted of any crime -- was denied a request to be released to campaign.

The vote was considered a litmus test of public opinion as it was the first poll since the Red Shirt protests ended in bloodshed.

The Reds, who hail Thaksin`s policies for the poor, view the current government as elitist and undemocratic because it came to power after a court ruling threw out the previous administration.

BDST: 19:31 HRS, July 25, 2010