Bystanders, security personnel and journalists raced to take cover in a north Bangkok shopping mall after a man pulled an assault rifle from a bag and began spraying bullets during a stand-off between government supporters and scores of opposition demonstrators, an AFP reporter at the scene said.
The firing went on for at least one hour.
Emergency workers said several people have been injured in the fighting, which broke out as anti-government groups laid siege to a ballot box distribution centre in the Thai capital.
"One victim was apparently shot in the chest and was hospitalised," an official from the city's Erawan emergency centre said, adding that two others had also been taken to hospital.
Tensions are high in the capital ahead of controversial elections on Sunday, which opposition demonstrators have vowed to block as they seek to prevent the likely re-election of Prime Minister Yingluck Shinawatra.
Bangkok has been rocked by weeks of sometimes bloody rallies by a loose coalition opposed to Yingluck and the enduring influence of her brother Thaksin Shinawatra -- a former premier ousted by the military in 2006.
The unrest is the latest round of political instability to hit Thailand since royalist generals ousted Thaksin seven years ago, unleashing a cycle of occasionally-violent street protests.
Saturday's clashes happened after demonstrators blocking ballot boxes from being delivered from the Lak Si district office in northern Bangkok -- one of 50 in the capital -- were confronted by a group of some 200 government supporters, some armed with sticks and metal bars.