An explosion and gunfire rang out near a sprawling anti-government protest site in the Thai capital early on Tuesday after the protesters' leader warned that government supporters were planning to bring armed militants to Bangkok.
Weeks of unrest, in which protesters have barricaded several main Bangkok intersections, have been interrupted by occasional bombs and gunfire, with one blast killing a woman and a young brother and sister in a shopping district on Sunday.
There was another explosion and gunfire near one protest site on the edge of Bangkok's Lumpini Park in the early hours of Tuesday, national security chief Paradorn Pattanathabutr told Reuters.
Two men were wounded, medical sources said.
"Last night, we don't know where and who it came from, but there was an explosion and the sound of gunfire from 1 a.m.," Paradorn said.
"Officials will investigate the area this morning and there should be more information soon."
He also said there was an explosion near the office of the opposition Democrat Party. No one was hurt.
The protesters, who disrupted a general election this month leaving the country in political limbo, aim to unseat caretaker Prime Minister Yingluck Shinawatra and erase the influence of her brother, former premier Thaksin Shinawatra, seen by many as the power behind the government.
Protest leader Suthep Thaugsuban accused Jatuporn Promphan, a leader of the "red shirt" supporters of Thaksin, and of Yingluck's government, of wanting to bring armed militants to Bangkok from their power base in the mainly rural north and northeast, setting the stage for potential conflict.
He also accused police of doing nothing about it.
"It is clear that Jatuporn wants to divide the country in two," Suthep told supporters late on Monday.
Suthep said protesters would target Shinawatra businesses again on Tuesday, a threat that sent stock prices tumbling last week.
But some shares were recovering on Tuesday.
SC Asset Corp, a property developer controlled by the Shinawatra family, lost almost 10 percent in the second half of last week and mobile handset distributor M-Link Asia Corp, also with links to the family, lost 12 percent.
SC Asset was up 1.3 percent at 0356 GMT and M-Link up nearly 7 percent.
National flag carrier Thai Airways International releases 2013 results on Tuesday and is expected to report a huge loss. It may cite a slump in tourism since the protests began last November as one of the factors.
Trade figures for January could show the biggest fall in imports since the global financial crisis as consumption, construction and other activities weaken.
At least 20 people have been killed and more than 700 wounded since the protests began in November.
It is the worst political violence since 2010, when Thaksin's supporters paralysed Bangkok for weeks.
More than 90 people were killed and 2,000 wounded during that unrest, which ended when Suthep, then a deputy premier, sent in troops.
Demonstrators accuse former telecoms tycoon Thaksin of nepotism and corruption and say that, prior to being ousted by the army in 2006, he used taxpayers' money for populist subsidies and easy loans that bought him the loyalty of millions.