Protesters block roads in Bangkok
It was part of a months-long campaign to thwart elections and overthrow the democratically elected prime minister.
The intensified protests, which could last weeks or more, were peaceful and life continued normally in much of the capital.
But they raise the stakes in a long-running crisis that has killed at least eight people in the last two months and fuelled fears of more bloodshed to come and a possible army coup.
Overnight, an unidentified gunman opened fire on protesters camped near a vast government complex, shooting one man in the neck who was admitted to a nearby hospital. The drive-by was the third of its kind since January 6.
In a separate incident early today, a gunman fired about 10 shots at the headquarters of the opposition Democrat Party, shattering several windows but causing no casualties.
The protesters are demanding that prime minister Yingluck Shinawatra's administration be replaced by a non-elected "people's council" which would implement reforms they say are needed to end corruption and money politics.
Critics have lashed out at the moves as a power struggle aimed at bringing the south-east Asian nation's fragile democracy to a halt.
Candlelight vigils have been held to counter the shut down and urge the February 2 election to be held on schedule despite an opposition boycott.
In a speech late yesterday, protest leader Suthep Thaugsuban repeated a vow that neither he nor his supporters will negotiate an end to the crisis.
"In this fight, defeat is defeat and victory is victory. There is no tie," he said. "The masses from all walks of life have woken up. They're aware that we are the owners of Thailand."
Protesters have vowed to surround Cabinet ministries to prevent them from functioning, and vowed to cut water and electricity to the private residences of Ms Yingluck and her Cabinet.
Most Thai and international schools in Bangkok were closed today, as were some major shopping malls. Many residents appeared to stay home, and traffic was light across much of the city.
The protests centred on seven major intersections, where demonstrators cut roads with walls of sandbags or vans and organised lively sit-ins on mats beneath stages equipped with speaker systems.
The crisis dates back to 2006, when mass protests calling for then-prime minister Thaksin Shinawatra - Ms Yingluck's brother - to step down because of alleged corruption and abuse of power led to a military coup.
Since then, supporters and opponents of Mr Thaksin have vied for power, sometimes violently.
The protesters say that billionaire Mr Thaksin, who lives in exile, continues to manipulate Thai politics. He commands overwhelming support in Thailand's less well-off rural areas, where voters are grateful for his populist programmes, including virtually free health care. He and his allies have won every national election since 2001.
Protest leaders have said they will maintain their "shutdown" for weeks, or until they obtain their goal. Their recent demonstrations have drawn up to 150,000-200,000 people at their height.