A jury in Manhattan was asked on Friday to decide whether an Occupy Wall Street activist intentionally elbowed a New York police officer in the face, or might have reacted instinctively to having one of her breasts grabbed from behind.

Making their closing arguments at the end of a three-week trial, state prosecutors and lawyers for Cecily McMillan painted sharply contrasting pictures of the night of 17 March 2012, when she is alleged to have assaulted Officer Grantley Bovell.

Bovell, a 35-year-old who usually patrols the Bronx, alleges that McMillan deliberately struck him as he led her out of Zuccotti Park, in lower Manhattan, where protesters had been marking six months of the Occupy movement. McMillan, a 25-year-old graduate student at the New School, denies the charge. She faces up to seven years in prison if convicted.

"There is no proof beyond a reasonable doubt," Martin Stolar, McMillan's lead attorney, told the jury at Manhattan criminal court, during an exhaustive speech lasting two hours and 40 minutes. "So the only verdict that you can reach is not guilty. Send Miss McMillan home. Send her back to school, let her finish her thesis and move on, and become a teacher, or a politician, or president of the United States."

Accusing McMillan of trying to use Occupy as a shield, however, Erin Choi, an assistant district attorney, said: "It is time for the defendant to answer for her own criminal actions. Our founding fathers did not create a right to free assembly so people could commit crimes and hide behind their right to protest. This is a sacred right that should be preserved and protected."

Stolar told the jury that on a "day off from protest", his client became caught up in the chaotic scenes at Zuccotti Park, which police were clearing for cleaners, after she stopped by to collect a friend to continue St Patrick's Day celebrations that saw her dressed in bright green. "Happy St Patty's!!!," she wrote to her father earlier that evening, in a text message shown to the jury.

Stolar said that Bovell had been unable to explain an allegation that McMillan had gone from angrily remonstrating with a female police officer, who he said had been asking her to leave the park, to being calmly led out by Bovell – "a hallmark, you might think, of somebody who is making something up," said Stolar.

He stressed that the supposed female officer, the only other apparent direct eyewitness to the clash, had never been identified or found. "I suggest to you that if that female police officer existed, she would have been found. If a serious attempt had been made to find that female police officer, it would have happened."

Choi reminded the jury that Bovell had testified that after "turning to her left and saying: 'Are you filming this?', 'Are you filming this?'" while being led out, McMillan "crouched down, then bent her knees, and then aimed her elbow at the officer and then jumped up to strike". Photographs showed that the officer suffered a black eye. He said that he went on to feel sensitivity to light and headaches.

Stolar said: "By his testimony, the woman in the bright green dress, not someone hidden behind a mask, not someone who's wearing dark clothing, who stands out and can't be missed, says: 'Film this, I would like to have a record of me in my green dress committing a crime – that's what I want. Please film this'. That's his testimony. Does it make sense? Really not too much.

"It just doesn't make sense, because it didn't happen the way that Officer Bovell says it happened," Stolar went on. Instead, he said, "she's on her way out, when somebody – and she doesn't know who when it occurred – grabs her on her breast, and the next thing she knows, she is down on the ground." He said McMillan had later suffered post-traumatic stress disorder.

Choi described McMillan's account as "so utterly ridiculous and unbelievable that she might as well have said that aliens came down that night and assaulted her". She dismissed claims made by the 25-year-old on the witness stand to not clearly recall the incident.

"She conveniently forgets her criminal conduct and how she tried to evade responsibility for that conduct," said Choi. Attempting to explain McMillan's remarks moments before the clash, the prosecutor said: "She wanted to make sure there was an audience who would watch her assaulting a police officer."

The prosecutor showed grainy video clips of the incident, downloaded from YouTube, which she said proved that McMillan was not grabbed, and instead decided to strike Bovell. "The fact that she thought she could convince you that that was someone grabbing her breast is absolutely offensive," she said. Stolar said that the footage was not clear enough to be decisive.

Wielding an enlarged copy of a photograph showing bruising to McMillan's chest, however, he said: "This is, to the extent that there is one, the smoking gun in this case. This is actual physical evidence that backs up Miss McMillan's account of what took place – that she got grabbed, and she reacted."

Choi, however, noted that the bruising had not been detected during two hospital checks on the night of the incident and suggested McMillan had caused it herself. She claimed that Bovell would have needed "razor blades for fingernails" and "a hot iron for a hand" to have caused her injuries. "You don't need a doctor to tell you these things are fabricated," said Choi. "You just need to use your common sense."

Both sides assailed the integrity of each other's key witness. Choi claimed that McMillan had "attempted to pervert this trial with grandstanding and outright falsehoods" and alleged that she had faked a seizure after her arrest. "Her character is someone who says one thing and does the complete opposite," she said. "Officer Bovell, by contrast, is someone who served his country in the navy for four years and then has served the city for nine years."

Stolar tried to focus the jury's attention on to Bovell's own admission that he had fixed five parking tickets for family and friends as part of the wider "Bronx ticketing scandal" that became public in 2011. "This is a police officer who, from almost the beginning of his career, has made it a habit of disregarding the law – of saying the law doesn't apply to me," said Stolar. Choi said Bovell had since been punished and accepted responsibility for his wrongdoing.

McMillan's attorney returned again and again to testimony from a series of character witnesses who claimed that she was a moderate activist. They said that from her very first encounter with Occupy protesters in the summer of 2011, she had made herself unpopular with more radical peers by proposing a course of peaceful engagement with the political system.

"Everybody agrees that Cecily's position, and what she's known for, is being a non-violent person, and not wanting to confront the state, but to work in collaboration with the state," said Stolar. "That means you don't go around slugging cops. That's not how you make political change. She is not going to punch out a cop. It serves no purpose for her politically, it serves no purpose for her personally. She is not someone who goes around hitting cops. It just is not her."

Choi, however, requested that the jury set aside their feelings on Occupy Wall Street and on McMillan's political ideology, and focus their attention instead on the two-second incident that resulted in the 25-year-old's elbow making contact with the police officer's face.

"No one is suggesting that the defendant believes in the violent overthrow of governments," said the prosecutor. "It's nice that she doesn't believe in burning down buildings, but it has nothing to do with this case, and nothing to do with the fact that she assaulted a police officer."

The jury is scheduled to receive instructions from Judge Ronald Zweibel on Monday morning, before being sent out to consider their verdict.