Protesters occupying two airports in Bangkok are bracing for a battle with security forces on Friday.
Thailand's prime minister has declared a state of emergency to end a blockade threatening to cripple the economy.
The People's Alliance of Democracy has placed a series of road blocks of razor wire, crash barriers and plastic water bottles on an expressway leading to Suvarnabhumi airport, which they have occupied since Tuesday.
All vehicles entering the area are being checked by protesters.
Police have begun talks with the protesters, saying they will negotiate with the demonstrators to avoid a bloody end to the siege, but the protesters say they are prepared to defend themselves "to the death" against any police operation to dislodge them.
There were similar scenes at Don Muang airport which was closed late on Wednesday.
In a televised address on Thursday, Prime Minister Somchai Wongsawat declared emergency law.
However, a similar declaration in September to dislodge protesters occupying Government House was ignored by the army.
The airport occupations have forced hundreds of flights to be cancelled, stranding thousands of foreign tourists. Suvarnabhumi handles 125,000 passengers a day.
Rumours of a military coup are swirling around Bangkok and imminent developments are expected, reports the Canadian Broadcasting Corporation.
Thailand has been in a state of political stalemate since former prime minister Thaksin Shinawatra was ousted in a military coup in 2006.
Fresh elections at the end of 2007 failed to resolve the crisis, when a party made up of former allies of Mr Thaksin returned to power.
The People's Alliance of Democracy, a loose alliance of royalists, businessmen and the urban middle class, claims that the government is corrupt and hostile to the monarchy.
They also accuse it of being a proxy for Mr Thaksin, who remains popular among Thailand's rural poor.
No cause for concern - Key
Prime Minister John Key says the Government is watching events in Thailand closely, but does not think there is any cause for concern for New Zealanders in the country.
Mr Key says the New Zealand embassy is in contact with New Zealanders in Thailand.
He still plans to go to the East Asian Summit to be held in Thailand next month.