The political rivals clashed with pistols and assault rifles despite tight security put in place to thwart the opposition's attempts to scuttle the vote, including the deployment of over 2,00,000 security personnel.
Soldiers were reported to be moving into Lak Si area, where the clashes erupted, to assist police in controlling the situation.
Two explosions were heard in the area, which police said were caused by Molotov cocktails, before the firing began.
People caught up in the violence took shelter inside a nearby mall and a covered pedestrian bridge, while others were seen hiding behind vehicles, Bangkok Post reported.
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Protesters marched in Bangkok and laid siege to a building where ballots were stored in a final bid to derail the polls and stop beleaguered premier Yingluck Shinawatra from returning to power.
A total of 49 million voters are eligible to exercise their franchise. Police and army personnel will provide security at more than 93,000 polling stations nationwide.
The CMPO would use its authority under an emergency decree to ensure smooth polling, Chalerm, also the caretaker Labour Minister, said referring to the 60-day emergency imposed last week in Bangkok and nearby areas to tackle months of unrest.
Unfazed by threats from political rivals to block the vote, Yingluck today rebutted the opposition Democrat Party's claim that the election was "unconstitutional".
"What does unconstitutional mean? The 2007 charter, particularly the section on election regulations, was altered by the Democrat-led government, not this administration," she said.