On this much, both sides agree: It was a tragic, chance encounter on a quiet street here last spring that left 18-year-old Michael Colono dead and Harvard graduate student Alexander Pring-Wilson accused of murder for stabbing the local Cambridge man five times with a pocketknife.

But at the start of a trial that has sparked talk of town-vs.-gown tension in this diverse academic hub, prosecutors and defense attorneys offered starkly different versions of the events of April 12, 2003.

Assistant District Attorney Adrienne Lynch said that Pring-Wilson "acted out in anger, using the knife repeatedly on an unarmed teenager," and added that his actions "were not in self-defense."

But Rick Levinson, who represents the 26-year-old defendant, said his client was the victim of a brutal attack as he made his way home early that Saturday morning from a local nightclub, and that Pring-Wilson's actions "while tragic, were justifiable."

Local coverage has focused on the divergent backgrounds of the two young men involved, whose friends and family members packed the court on Monday. "You will learn about the very different paths each of their lives took, intersecting in one fateful minute and 10 seconds," Lynch said, referring to the alleged duration of the fight.

Pring-Wilson, who sat impassively in court in a gray suit while the incident was described, is on a leave of absence from Harvard's Davis Center for Russian and Eurasian Studies, a university spokesman said. He was raised in Colorado Springs, the son of two prominent attorneys.

Colono was working nights at a Boston hotel and had a 3-year-old daughter. After dropping out of high school he had brushes with law enforcement, including drug violations, according to court records. But family members said he recently earned his General Educational Development diploma and was starting to turn his life around.

According to court documents, Pring-Wilson was on his way home from a nightclub just before 2 a.m., when he walked past a white Chevrolet Celebrity where Colono was sitting with his cousin Samuel Rodriguez and Rodriguez's girlfriend Giselle Abreu. Abreu testified Monday afternoon.

Many of the case's other details are disputed. Lynch said that when Colono mocked Pring-Wilson for staggering and appearing drunk, the student opened the car door and the men began to fight. Pring-Wilson, who was taller and heavier, soon "had the upper hand," Lynch told jurors Monday.

But Levinson said his client was walking away when Colono and Rodriguez jumped him, wrestled him to the ground and began punching and kicking him in the head.

Pring-Wilson allegedly stabbed Colono in the chest, stomach and arm with a military folding knife he always carried with him. Levinson said that in Colorado, carrying such a knife is not uncommon and that when Pring-Wilson used it that night, "the sole purpose was to stop the beating that they were administering to him."

Eventually, Colono and Rodriguez returned to the car and left the scene. When Colono began to have trouble breathing, he realized he had been stabbed and his friends rushed him to get medical attention, court records said. He died in a Boston hospital less than two hours later.

Prosecutors said Pring-Wilson gave several versions of the events in the first hours after the incident. In a 911 call he said he had witnessed a stabbing, and later he told police he had helped break up a fight before finally admitting he fought with Colono.

Police also obtained a voice-mail message Pring-Wilson left for his friend Jennifer Hansen, who was with him earlier at the nightclub. In the message, Pring-Wilson said, "I just got attacked by a group. I fended them off. I stabbed him a couple of times, and don't repeat this to the police."

Levinson said Colono's friends have also changed their story, initially telling police that Colono left the car on his own and threw the first punch.

Pring-Wilson was originally denied bail, but it was later set at $400,000, and he was confined to house arrest at an apartment in Somerville, Mass. Pring-Wilson's attorneys had earlier attempted to have the venue changed, saying he could not get a fair trial in Cambridge. The 15-member jury was selected last week, and Emily J. LaGrassa, a spokeswoman for the district attorney's office, said the trial should last about a month.

In preliminary hearings and court filings, Pring-Wilson's attorneys have stressed his unblemished legal record, and his friends and colleagues described him as a fun-loving, intelligent young man who went by the nickname Xander. A compilation of testimonials about his character was presented during a bail hearing in May.

"He was, in a word, wonderful. He was the kind of student that makes a class come alive," said John Riker, a philosophy professor at Colorado College, where Pring-Wilson studied as an undergraduate.

But Riker added that Pring-Wilson was not one to back down from a confrontation. "He was also a rugby player. He liked to mix it up and had this sense to him that 'I won't attack you but don't tread on me. I'm going to stand my ground,' " Riker said.

During his opening statement Monday, Levinson described Pring-Wilson as an accomplished student and world traveler and cited Rodriguez's criminal record, including prior convictions for assault. Colono's family members have expressed frustration at what they say is an attempt by defense attorneys to use Pring-Wilson's Ivy League pedigree to argue that he did not commit the crime.