Pol Gen Adul made the apology at a press conference at the Centre for Administration of Peace and Order (Capo).
He admitted police officers who vigorously smashed in the windows of a health volunteer’s car on Dec 26, when anti-government protesters were blocking poll registration, had overreacted.
The police chief said the police that day were under pressure from the demonstrators, and reacted after shots were fired at them.
Pol Gen Adul defended his long delay in telling the truth, saying he needed to have all the facts before reaching a conclusion on the case.
Capo spokesman Pol Maj Gen Piya Uthayo on Dec 28 denied police had damaged the vehicle and blamed the incident on "protesters who disguised themselves as policemen with bullet-proof vests stolen from a police car".
The national police chief admitted for the first time on Thursday that police were the "men in black" on the rooftop of a Labour Ministry building and that riot police had also trashed the health worker's vehicle.
The admission came after protest leader Suthep Thaugsuban on Wednesday night on the protest stage demanded answers from top police officers, including Pol Gen Adul, deputy national police chief Pol Gen Worrapong Chiewpreecha and Metropolitan Police Bureau commander Pol Lt Gen Kamronwit Thoopkrachang.
The People's Democratic Reform Committee leader gave them a week to disclose the investigation results to the public.
Protest leaders have denied planning violent rallies against caretaker Prime Minister Yingluck Shinawatra, even while demanding she and her cabinet step down to allow reform before a general election. They have accused police of initiating the violence.