Mr Bo said he was coerced into making a confession and rubbished testimony from witnesses who included his wife.
The former Chongqing Communist Party boss is also charged with corruption.
He faces charges of abuse of power relating to his wife’s role in the murder of a British businessman.
Two years ago the 64-year-old high-flier was seen as a candidate for promotion to the Politburo Standing Committee, China’s seven-member top decision-making body.
But in February 2012, as China prepared for its once-in-a-decade leadership handover, questions emerged over the death of a British businessman, Neil Heywood.
Mr Bo’s downfall was seen as the biggest political shake-up to hit China’s ruling elite in decades. His wife, Gu Kailai, has since been convicted of Mr Heywood’s murder.
Correspondents say the trial is as much about getting rid of a popular politician as it is about criminal wrongdoing. Mr Bo is widely expected to be found guilty.
Events in the courtroom gripped the nation. “Bo is a psychologically tough man. This kind of person is very appealing. I don’t think he will plead guilty easily,” one person wrote on China’s Twitter-like Sina Weibo
The trial is taking place at the Intermediate People’s Court in Jinan, in eastern Shandong province. The court is providing an account of proceedings on its official microblog.
“I hope the judge will try this case fairly and justly according to the law of the country,” Mr Bo reportedly said.
Five of Mr Bo’s relatives are said to be in court, along with 19 journalists and 84 other people. Foreign journalists have not been allowed in.
According to the indictment posted by the court, Mr Bo is accused of receiving bribes totalling 21.8m yuan ($3.56m; £2.28m) from two Dalian-based businessmen. He is also charged with
Mr Bo went on the offensive, denying accepting any money in the form of bribes.
The court microblog quoted him as saying about one of the cases: “That [businessman] Tang Xiaolin gave me three bribes – that did not happen. He asked me to help him sort out something, and these were all done according to procedure.”
Mr Bo said he had admitted taking three bribes from Mr Tang “against his will” under questioning, adding: “What I meant was that I was willing to take the legal responsibility but I had no idea of all these details back then.”
Using unexpectedly strong language, he described Tang Xiaolin as a “crazy dog” who was “selling his soul” to reduce his sentence, which brought a rebuke from the judge.
Mr Bo said accusations he had taken bribes totalling more than 20 million yuan from a second businessman, Xu Ming, were “completely untrue”.
He said that he “knew nothing” about a property owned by his wife near Nice in France or about Xu Ming sponsoring his son Bo Guagua’s schooling.
“Gu Kailai only told me that Xu Ming is kind to Guagua – that’s all,” he told the court. “Xu Ming was Gu Kailai’s friend and not mine.”
In an afternoon full of surprises, Mr Bo was allowed to cross-examine Xu Ming. Asked if he had told the politician about funding a trip by Bo Guagua to Africa, or buying expensive presents for him and Gu Kailai, the businessman repeatedly replied “no”.
Xu Ming is in custody. Tang Xiaolin’s whereabouts are unclear.
Mr Bo reserved some of his strongest remarks for his wife’s testimony, which he called “ridiculous”. In written evidence provided by prosecutors to the court, Gu said she had seen a large amount of money in the family’s safes matching the amount allegedly given to Mr Bo by Mr Tang.
“How could she say for certain that I put the $50,000 or $80,000 into the safes?” he asked.
The court session was adjourned until 08:30 (00:30 GMT) on Friday.
Hearings would last two days, Chinese state television CCTV said earlier in a tweet, with a verdict “likely in early September”.
Security was tight at the court, with police blocking the gates and lining roads leading up to it.
According to the court indictment, the corruption charges against Bo Xilai relate to the alleged embezzlement of public money in 2002.
“While serving as governor of Liaoning Province, Bo used his post to conspire with others to embezzle five million yuan of public funds from the Dalian government,” the charge sheet reads.
The abuse of power charge is connected to his wife’s role in Mr Heywood’s murder and his treatment of Wang Lijun, his now-jailed former police chief whose flight to the US consulate brought the case out into the open, the court indictment said.
As party leader in Chongqing, Bo Xilai was seen as a powerful, populist and charismatic figure.
He was known for two high-profile campaigns: a large-scale crackdown on crime and a drive to promote China’s old communist values. But analysts said his ambition earned him enemies and he was considered controversial by top party leaders.
In February 2012, around the time that China was preparing to promote a new generation of leaders, his police chief, Wang Lijun, fled to the US consulate in Chengdu amid an apparent fall-out with Mr Bo.
Shortly afterwards, Chinese authorities announced that they were reinvestigating the death of Mr Heywood, and both Mr Bo and his wife disappeared from public view.
Gu Kailai has since been jailed for the murder of Mr Heywood – a crime she carried out, state media say, because of differences over a business deal. Wang has also been jailed for his role in covering up events, among other charges.
Mr Bo is the last major player in connection with the case to face trial. Interest has been intense, with many Chinese microbloggers speaking out.
Most expressed scepticism about the proceedings, judging the trial merely a political show. Nevertheless a large number still believe Mr Bo has a case to answer when it comes to corruption and abuse of power.
Herkuang in Shanghai says: “This Bo Xilai trial thing is merely a procedural thing. Those [party] elders have already made up their minds on what to sentence him with… Just watch the end result.”
But some of comments about Mr Bo and his alleged conduct were dripping with sarcasm. “What a clean official! Just one count of bribe-taking in 30 years as a civil servant?” Li Zhiqiang, law lecturer at Lanzhou University, said.
Other internet users were more complimentary, however. Tencent Weibo user Yufan from Chongqing wrote: “I don’t know if he has actually taken any bribes. All I know is I like his calm face. It’s quite okay.”