Witnesses reported that unidentified assailants in a white pick-up vehicle fired gunshots and grenade launchers at a barricade outside a protester's camp.
Nine of the injured were hospitalised. One died while being treated.
The attack comes at a time of heightened tensions in a six-month political crisis that had already left 26 people dead and hundreds injured.
Thailand's first woman prime minister Yingluck Shinawatra was forced from office last week by a controversial ruling in the country's Constitutional Court relating to her demotion of a senior security official in 2011.
Pro-government Red Shirts have massed on Bangkok's outskirts since the ruling which they said was part of an orchestrated campaign by influential establishment figures in Bangkok to force Ms Yingluck's wealthy family to quit politics.
Red Shirt leaders have been urging their supporters not to react to the ruling, saying they fear an outbreak of violence could prompt the military to mount a coup.
The military, which has staged 18 coups or attempted coups since the 1930s, has repeatedly denied plans to intervene and urged political rivals to negotiate a settlement.
Witnesses said the anti-protesters' camp is now calm after the attack about 2.30am Thursday.
The protesters moved to Democracy Monument last weekend after months camping at Bangkok's central Lumpini park.
There have been 80 violent attacks in Bangkok since protesters took to the city's streets last November.
One attack late last week was on the home of a Constitutional Court judge.
Attackers have often used grenade launchers.
Thailand's Election Commission was due to meet with government leaders on Thursday to discuss the holding of fresh elections on July 20.
Elections held in February were annulled after anti-government protesters disrupted voting in some areas.
The election was boycotted by the opposition Democrat Party.
Thai authorities have issued warrants for the arrest of dozens of anti-protest leaders but police have not yet moved against them, apparently fearing this would provoke more violence.
The crisis is the latest chapter in a years-long struggle for power by rival groups of Thai elite, one backed by Bangkok's middle class and royalist establishment and the other by rural masses who support the Shinawatra family, including Ms Yingluck's elder brother Thaksin Shinawatra, a former prime minister and business tycoon who lives in exile to avoid a jail sentence for corruption.
Australia's smartraveller.gov.au warns Australians in Thailand to avoid protest rally sites.
One side cannot win an election, nor have they for some time. They are out numbered by their opponents. Now we are hearing their plan to set up committees to run the government. Non electable parties want forums to marshall the minority to lead the elected majority. The real transgression was a simple farm subsity for poor farmers with a price guarantee for their production. Nothing the United States of America has not done and at much cost rewarding a certain base. The opposition is losing but they can pull of an overthrow and take over an elected government that still has the support of the msjority of the countries people.
Posted by The Questioner on May 15, 2014 14:53
Editor Comment:
No mention of entrenched corruption and nepotism, The Questioner. Not issues for you?