Ji Ungpakorn, a political-science lecturer from Chulalongkorn University who has been charged with lese majeste at home, was in the audience.
He made some stinging comments during the question-and-answer session and used his foot-shaped clappers to greet Abhisit while other audience members applauded.
"I can't tell how fast the Thai democracy will progress. Looking back at the Western experiences, you'll find that it took centuries," Abhisit said.
"Most importantly, democracy in Thailand will no longer go backward. Thai people are now at the crossroads. Be assured that the people have taken the right path and will go ahead despite some obstacles," he said.
The PM was speaking to a full house at Oxford University's St John's College, where he gave a lecture on "Taking on the Challenges of Democracy" late yesterday morning.
The Thai News Agency reported that Abhisit told his audience Thailand's previous round of political reform, which resulted in the Constitution of 1997, had come to a halt when politicians in power abused certain provisions in the charter for their own benefit.
He blamed the abuse of power for the military's return to politics by staging the coup of September 2006.
The premier said it appeared to be in the "nature of Thai politics" to see some progress and then some reverses, referring to coups.
He said that a new round of political reform, which would bring about a stable democracy, would be based on the principle of good governance.
After Abhisit completed his speech, the red-clad Ji rose to comment that the Thai government was using the lese-majeste law to protect itself and the military.
The premier responded that the law could be compared to a defamation law for ordinary people, due to the fact that the monarchy had no legal channel to defend itself against defamation.
He pointed out that there were similar laws in European countries.
Abhisit said many cases had been brought to court and most defendants had not fled. At that point Ji retorted: "I didn't escape;" to which the PM said: "Then why can you be here now?" This drew applause.
Other audience members then asked questions, mostly about Asean, human rights in Thailand and the Rohingya boat people. Abhisit is scheduled to arrive back in Bangkok this afternoon.