Article published on the 2008-10-17 Latest update 2008-10-17 15:15 TU
As about 5,000 supporters of the People's Alliance for Democracy (PAD) rallied in Bangkok's business district, Somchai said that he would not quit.
"Whether I resign or do not resign will depend on national interest," he said in a televised statement.
Somchai took the job over when a court ruled that his predecessor, Samak Sundaravej, had to stand down.
On Thursday army chief Anupong appeared on television along with the heads of the navy, air force and police and said that, if he were Somchai, he would resign.
The military kicked out Somchai's brother-in-law Thaksin Shinwatra in a coup last year but analyst Giles Ungpakorn believes that the army chief has a different strategy today.
"He’s trying to put pressure on the Prime Minister to resign and I think what they’re after is that they want to see the parliament suspended," he told RFI. "They want a government of so-called 'national unity' but it won’t be represented by the people who’ve won the most votes in the country."
PAD demonstrators Friday carried placards declaring Somchai a murderer, in reference to the deaths of two of their supporters in clashes with the police on 7 October.
PAD leader Somsak Kosaisuk promised to distribute 100,000 books and CDs with photos and accounts of the violence, claiming that they would "show why the government has no legitimacy to run the country".
In neighbouring Cambodia, Prime Minister Hun Sen vowed to beef up the country's defence but insisted that there will be no war with Thailand over the disputed border area where two Cambodian soldiers were killed on Wednesday.