Al Jazeera's Aela Callan, reporting from Bangkok, said that the number of protesters in the business district had dwindled to about 5,000 people and there had been a significant build up of troops outside the barricades surrounding the site.
"Information very sketchy as the fighting is going on all around not just in the protest area," she said.
Red shirt checkpoint
The army had moved on a red shirts at a checkpoint set up outside the main camp on Friday after a series of violent clashes left at least one person dead and nine injured overnight.
Military vehicles and a bus were reportedly set on fire as the red shirts attempted to halt the troops.
The protesters had gathered outside the Suan Lum night market to stop soldiers from advancing towards the main site.
Electricity has been cut off to that part of the capital in an apparent attempt to force the protesters out and sections of the city's elevated rail system were closed, including at Sukhumvit Road, a key tourist area.
"Bangkok is under a state of emergency," Al Jazeera's Wayne Hay, reporting from the protest site, said.
"So things are stepping up and the government is trying to increase pressure on the red shirts.
"There is still a hardcore of protest leaders there who are staying put and getting ready for a fight."
He said that the military pressure may work as a "wake-up call" to moderate protesters to go home, but that some people were afraid to leave.
The military has said that snipers could be deployed to the area and armoured vehicles may also be used around the site to prevent more people joining the about 10,000 protesters in central Bangkok.
Renegade general killed
Clashes erupted late on Thursday after a suspended army general allied with the red shirt movement was left in a coma after being shot in the head.
Witnesses said the shooting was apparently carried out by a sniper, but it was not clear exactly who was behind the attack.
Panitan Wattanaygorn, the acting Thai government spokesman, told Al Jazeera that soldiers were not involved.
"Our operation is to secure the outer areas of the demonstrations. We will investigate as the red shirts have problems with their own leaders. Our officer was killed in a similar way in the last week," Panitan said.
But Phongthep Thepkanjana, a red shirt ally and former minister of justice, told Al Jazeera: "I don't think anybody can accept that because there was information that the government has snipers deployed close to the demonstration site.
"He was shot from a long distance."
The former army general was speaking to a group of journalists as he was shot inside the barricades. He is said to be in a coma and fighting for his life.
"He is one of the more hardline of the red shirt group and had been criticised for being too radical," Al Jazeera's Callan said.
"It is hard to say what exact effect this will have on the protesters but it will certainly bring down morale as he is one of their fighters, but he is not one of the rank and file, one of the core leadership group. He is not making decisions so it is more a symbolic loss.
Street rallies have been held since March 12 in an attempt to bring a disolution of parliament and fresh elections.
The red shirts have called the government illegitimate as it came to power in a 2008 parliamentary vote.
This followed a controversial court ruling that ousted elected allies of Thaksin Shinwatra, an former prime minister who was himself unseated in a 2006 coup and remains closely linked to the red shirts.
Abhisit Vejjajiva, the Thai prime minister, on Thursday held up a plan to hold early elections in November as deomonstrators refused to clear from the city centre.
The red shirts have said that they will not leave until Suthep Thaungsuban, the deputy prime minister, is charged for his alleged role in a deadly crackdown on protesters on April 10.
Demonstrations have often brought parts of central Bangkok to a standstill for long-periods and have included clashes with police that have left 29 people dead and more than 1,000 injured.
The red shirts initially called for immediate elections but agreed to a reconciliation deal this month in which Abhisit offered to dissolve parliament in the second half of September and hold polls on November 14.