UPDATE 3 (19.50 h CEST):
Police have released the five suspects in the five bombings at Silom intersection on Thursday night.
One of the suspects, Jaturon Noppakit, 36, was a red-shirt supporter of the anti-government United Front for Democracy against Dictatorship (UDD).
Mr Jaturon said he was travelling from Wongwian Yai to join the red-shirt protesters in Silom but insisted he was not involved with the bomb attacks.
The other four were supporters of the multi-coloured group who opposed the red-shirt's movement. They had digital cameras and video cameras containing footages of the soldiers working at Silom intersection.
"Police release 5 supsects in Silom blasts", Bangkok Post, April 22, 2010
UPDATE 2 (19.30 h CEST):
A series of grenade blasts that hit Bangkok's business district on Thursday killed at least three people, Deputy Prime Minister Suthep Thaugsuban told reporters. He also said the government had no plan to crack down on anti-government protesters in the area, because women and children are among them.
"Bangkok grenade blasts kill 3, deputy PM says", Reuters, April 22, 2010
Deputy Prime Minister Suthep Thaugsuban said in a national television broadcast at 11.30pm that the grenades were fired from behind the monument of King Rama IV, at the entrance of Lumpini Park, where the red-shirts were gathering. "They were fired from the red-shirts' side," he said.
He told the pro-government group, who called themselves the Love Silom group, to retreat.
(...)After the explosions, the two rival groups clashed again about 10.45pm near the Dusit Hotel. Both sides were seen hurling bottles at each other. Security forces reportedly had a difficult time controlling the area, according to television reports.
"3 killed, scores injured in Silom attacks", Bangkok Post, April 22, 2010
UPDATE 1 (19.00 h CEST): Reuters now reports three dead.
Shortly after the blasts occurred an angry mob is reported to have attempted to storm the red shirts barricade, but have been stopped by a line of police, which then was attacked, too.
A series of grenade blasts that rocked Bangkok's business district on Friday killed at least one person and wounded 75, hospitals and the Thai media said.
Five M-79 grenades hit an area packed with heavily armed troops and studded with banks, office towers and hotels. Four of the wounded had serious injuries, including two foreigners, according to witnesses, hospital officials and an army spokesman.
"Bangkok blasts kill one, injure 75 - Thai media", Reuters, April 22, 2010
This was the third night after the red shirt protestors have barricaded the intersection with bamboo fences, closing a road leading to the rally site. There have been standoffs between police, red shirts and pro-governments "no colors" protestors (while it is possible that man of them are yellow shirts in disguise) in the last few days at that very same spot. See Richard Barrow's map for detailed location.
Gregoire Glachant has video of the aftermath after the first blast. You can see one injured foreigner (probably Australian), also it shows a hole in the roof of the BTS Skytrain station.