The assault was carried out by two assailants who shot out the tyres of Sondhi's car and then riddled the vehicle with bullets, Panthep said.
Doctors later said Sondhi was out of danger after a successful operation to remove a bullet fragment from his skull.
Chaiwan Charoenchoketavee, director of Vajira Medical College, said: "The operation has been completed. It took around two hours and Sondhi is now safe, in a good condition and able to talk."
Al Jazeera's Tony Cheng, reporting from Bangkok, said the attackers in a pickup truck fired about 100 bullets on Sondhi's vehicle over five minutes.
Tight security across Bangkok is making it difficult for protesters to regroup on the streets and that may lead to more such targeted attacks, our correspondent said.
Cheng said the latest violence had heightened fears that the situation may return to how it was in the 1970s, when there were many factions, violence was prevalent and the country was deeply divided.
The PAD was not part of the latest protests in Thailand but bands of its yellow-shirted supporters reportedly clashed with the Red Shirts on Monday night.
Bangkok is still under a state of emergency despite the Red Shirts dispersing on Tuesday.
They had besieged the Government House offices of Abhisit Vejjajiva, Thailand's prime minister, for several weeks in an attempt to force him from office and prompt an election.
Thaksin, who continues to wield considerable political influence in the country despite being mostly in exile since being ousted in a military coup in September 2006, had called on his supporters to overthrow the government.
|The attackers apparently fired on Sondhi's car for five minutes, discharging about 100 bullets
But on Thursday, he urged his supporters to remain peaceful and echoed calls by Abhisit for reconciliation.
"War will never end by war, it has to end by negotiation," he said in Dubai.
"If the government wants to reconcile, I will encourage the Red Shirts to participate."
The former prime minister also urged King Bhumibol Adulyadej to help resolve the crisis that has deeply divided the country.
Sondhi founded the PAD in 2005 after falling out with Thaksin, who used to be a business associate.
The group is a loose alliance of royalists, military and urban elites who all oppose Thaksin, who draws his support from the rural poor.