Protesters trying to topple Thailand's government have moved to tighten the blockade around ministries in Bangkok.
A hardline faction has also threatened to storm the stock exchange, while major intersections in the capital remain blocked.
The turmoil is the latest chapter in an eight-year conflict pitting the Bangkok-based middle class and royalist establishment against the mostly poorer, rural supporters of Prime Minister Yingluck Shinawatra and her brother, Thaksin Shinawatra, a former premier ousted by the military in 2006.
Although the capital was calm and the mood among the tens of thousands of protesters mostly festive, analysts say the scope for a peaceful resolution of the crisis was narrowing.
"As anti-government protesters intensify actions, the risk of violence across wide swathes of the country is growing and significant," an International Crisis Group report said.
Ministries and the central bank have been forced to operate from back-up offices after protesters led by Suthep Thaugsuban stopped civil servants getting to work.
"In the next two or three days we must close every government office," Suthep told supporters.
"If we cannot, we will detain the prime minister and other ministers.
"We will start by cutting water and electricity and their homes.
"I suggest they evacuate their children."
Groups of demonstrators marched peacefully from their seven big protest camps to ministries, the customs office, the planning agency and other state bodies, aiming to paralyse the workings of government.
Stock exchange targetted
A student group linked to Suthep's People's Democratic Reform Committee (PDRC) has threatened to attack the stock exchange.
Faction leader Nitithorn Lamlua told supporters it represented "a wicked capitalist system that provided the path for Thaksin to become a billionaire".
A PDRC spokesman says the bourse is not one its targets.
"We will not lay siege to places that provide services for the general public, including airports, the stock exchange and trains," Akanat Promphan said.
"However, we will block government offices to stop them from functioning."
Security has been stepped up around the stock exchange but trading remains normal.
ABC's Samantha Hawley describes the scene:In the centre of Bangkok, gridlocked traffic has been replaced with a sea of humanity. It’s a surreal sight, because while the crowd stands shoulder to shoulder with barely room to move.
The anti-government protestors have set up makeshift camps on the street and from large stages, protest leaders spend the day revving them up via loudspeaker.
There is nothing but disdain among this group for the current Prime Minister, Yingluck Shinawatra, but there’s almost a carnival type atmosphere to this as well.
What hits you most is the colour and noise. The crowd is awash with red, white and blue. Large Thai flags are waved high in the air, music blares from loudspeakers aboard trucks carrying protest leaders and then there are whistles. The noise is deafening.
As the protest leader and former politician, Suthep Thaugsuban walks through the streets heavily protected by security he’s greeted like a hero.
And this is a boom time for Bangkok’s famous street food stalls that have multiplied to cater for the crowd.
So far the protestors are well mannered, and peaceful.
On day one there was no feeling of unease as you moved through the masses.
But as day broke the crowd was not as large and the carnival feel has certainly gone with the protestors marching to the gates of the police headquarters with a message they're a forced to be reckoned with.
And there are concerning threats.
A radical student groups says it will take over the stock exchange and shut down air traffic control centres if the Prime Minister doesn’t resign by tomorrow.
And the longer the protestors remain on Bangkok’s streets without getting what they want, frustrations will undoubtedly grow.
Disruption to government services would compound the problems faced by Ms Yingluck, who dissolved parliament in December and called a snap election for February.
Now working from Defence Ministry facilities on the outskirts of Bangkok, she heads a caretaker administration that's limited and can't initiate policies that commit the next government.
Ms Yingluck invited protest leaders and political parties to a meeting on Wednesday to discuss an Election Commission proposal to postpone the election until May.
"I am asking for cooperation and request all sides to take part in reforms. Whatever needs to be reformed or fixed - be that to get rid of corrupted politicians - it can be a reality," she said.
"In what they are doing now, the country is at a standstill.
"I am not holding onto my role as a caretaker but am holding onto the rule of democracy which is owned by the people."
However, Suthep has repeatedly said he is not interested in any election.
He wants the government to be replaced by an unelected "people's council" that will change the electoral system as part of reforms that will weaken Thaksin's sway.
It is widely thought that if the agitation grinds on the judiciary or the military may be provoked to step in.
The military has staged or attempted 18 coups in 81 years of on-off democracy, although it has tried to stay neutral this time.
Seven big intersections were occupied by the protesters and other roads were blocked off.
Many schools have been closed from Monday to Wednesday as a precaution in case of trouble.
But shops and most private offices were open, even though many shoppers and commuters appear to be avoiding the city centre for now.
The government has deployed 10,000 police to maintain law and order, along with 8,000 soldiers at government offices.