At about midnight I got a call from a photographer friend who was at Sanam Luang keeping an eye on the pro-government crowd. ''Come now, right now,' he said. ''They are marching up Rajadamnoen.''
The moment he said that I knew something bad was going to happen. Going up Rajadamnoen could only mean one thing - they were going to challenge the People's Alliance for Democracy (PAD).
I got there a half hour later, and walked first through the PAD camp and lines. The mood in the PAD camp was serious and businesslike. All the men at the barrier wore helmets and carried baseball bats. Some had slingshots. Many had improvised new riot shields. They were waiting.
From a distance, the sound of the pro-government crowd could be heard as they approached. I hurried to leave the PAD lines and go out and behind the pro government crowd before I was caught in the middle.
I soon came across the vanguard, approaching on the other side of a thin double line of police with only riot shields and no weapons.
There were around 5,000 pro-goverment supporter. They started arriving since the previous day, trickling into Bangkok from across the north and north-east.
The pro-government crowd, many wearing red shirts and red headbands, marched quickly towards the Makkawan bridge and was met by a double row of police in full riot protection gear but without batons.
The police let the crowd through. Then they simply walked away, watching from a distance as the almost medieval battle erupted.
Some in the crowd, armed with two by fours and rods and slingshots, began to run towards the PAD's sparsely guarded outer perimeter. The PAD guards manning the perimeter ran and the crowd chased them, throwing aside metal barriers. The men were screaming with rage as they ran at the PAD. One car window was broken and the pro-government crowd roared.
But as the few dozen young men in the vanguard of the pro-government crowd neared the PAD just after 1am, the close ranks of the PAD suddenly roared and came running out in a full charge, plowing into the pro-government crowd who were not only outnumbered but also ill-equipped to defend against the charge.
Simultaneously a volley of gunfire erupted from the PAD, and at least one pro-government protestor fell immediately, eyewitnesses from the pro-government group said. But Thai media reported that a PAD member had been killed.
Another report quoted a hospital source saying that the man was from Nakorn Ratchasima in the north east - where most of the pro-government crowd was from.
A pitched battle ensued where the two rival mobs met, with many at the rear of the pro-government crowd running helter skelter as they realised the PAD had the upper hand and heard the gunshots.
After an initial shot, more shots were heard at random intervals as the struggle surged back and forth for about two or three minutes.
The shots were clearly coming from the PAD ranks. Bullets zinged into the trees above me and another journalist from Bloomberg; we were the only foreign journalists there.
At the gunfire, I shouted 'bullet'. Dan Ten Kate and I both ducked and ran with the crowd. Many of them were climbing over walls and trying to break into nearby buildings to get away.
There were several moments of panic, as I searched for a way to get out of what most of us thought was a rampaging PAD descending on us.
But the situation calmed momentarily then.
I saw a man with blood pouring from his leg being carried away.
At least 5 ambulances took away the injured, estimated by some sources at between 25 and 35. More ambulances were standing by.
The street was strewn with rubble from the battle. Broken glass and flower pots crunched under my boots.
The pro-government crowd appeared stunned at being shot at by the PAD. 'We brought sticks and knives to a gunfight,' one man said. Several women sat on the sidewalk with their heads in their hands.
The PAD has advanced several metres and now massed in the street in a show of strength, facing down the pro-government crowd.
A man on top of a truck in the pro-government crowd screamed into a microphone urging the men not to back off. 'More of us are coming,' he screamed.
At 1.30am a second wave of a few dozen pro-government supporters armed with two by fours and swords and other assorted weapons appeared.
This time, the police intervened, forming a line across the road between the two warring sides. Police officers shouted at the crowds to stand down.
A volatile calm followed as the ambulances arrived and ferried away the wounded. The pro-government crowd screamed over the police at the PAD.
The crowd had come from all over the north east and northern provinces, strongholds of the ruling People Power Party (PPP).
At around 2.45am over 100 soldiers arrived with riot control gear and no weapons. An army source confirmed on the phone that the army was not on alert, but four companies of riot control soldiers had been sent out. The men in camouflage fatigues and holding riot shields fell in to support the police at around 2.45am.
Big trucks then started appearing, carrying tent poles and canvas and stacked with cases of drinking water for the pro-government crowd.
By 3.30am the chance of another clash appeared to have diminished, but the mood remained dangerously volatile. One pro-government protestor was seen pinning a print out in Thai on a tree which read 'We have taken back Rajadamnoen.'
Last Tuesday the PAD - a coalition of royalists with significant backing from the capital's royalist-military-bureaucratic elite, and from wealthy business families, invaded a government TV station, shut down three airports in the south disrupting over 15,000 travellers for two days, and overran and occupied Government House.
The PAD is demanding that prime minister Samak Sundaravej and his Cabinet who they say are corrupt puppets of former premier Thaksin Shinawatra, resign.
Mr Samak has refused to do so, insisting that as an elected prime minister he will not cave in to a minority.
But the situation has been locked in a deepening stalemate, which broke last night in the mayhem on Rajadamnoen.