MTA menace Isaiah Thompson didn't want to stand clear of the closing doors — so he violently slashed the MTA Bronx subway conductor who had asked him to do so, prosecutors said Friday at the beginning of the suspect's assault trial.

"Shut the f— up! Don't tell me what to do!" Thompson screamed at the MTA worker before grabbing her and slashing on the left side of her nose, just below her eye, according to prosecutors.

"Those are the words the defendant, Isaiah Thompson, said to the (MTA worker)," Bronx Assistant District Attorney Dylan Flanders told the jury at the start of Thompson's trial in Bronx Criminal Court, as a sea of MTA employees glowered at the suspect from the gallery.

"I think he's a monster," Chris Drummond, an executive board member of Transport Workers Union Local 100 said at a press conference after the opening arguments. "We're hopeful that we see some justice for our sister who was brutally slashed and assaulted."

The subway conductor, a 36-year-old mother of two who had joined the MTA two years before the afternoon attack on June 10, 2025, had reopened the doors of her No. 6 subway train for Thompson, 29, at the Morrison Ave.-Soundview station, Flanders said.

"As she was closing the doors, she saw one last person running up the stairs," the prosecutor said. "Upon first seeing him, she actually reopened the doors and allowed him to enter. She's going to tell you that was the defendant, Isaiah Thompson."

Thompson stepped into the train, but immediately made contact with the train conductor, Flanders said.

"In response to her saying her routine announcement that the doors would be closing, this defendant who was about 10 feet away looked her again in the eyes and he said, 'Shut the f— up! Don't tell me what to do!'"

The conductor was alarmed by Thompson's behavior, but "remained calm," Flanders said. "She made a mental note of what the defendant looked like."

When her train reached the Whitlock Ave. stop, Thompson stepped out of the train, grabbed the conductor and "began slashing her with a sharp object, leaving her with a permanent scar across her face," Flanders said.

"You'll have the opportunity to see the scar on her face," he told the jury. "You'll have the opportunity to hear her tell you that the person that did this to her is this defendant seated right here, Isaiah Thompson."

Police confirmed that Thompson has been arrested 18 times for a variety of criminal offenses, mostly along the city's rail lines. Before he assaulted the conductor, his crimes ranged from jumping turnstiles to train surfing.

On May 29, 2019, he was arrested for flashing commuters while riding on the side of a No. 2 train. When he got inside the subway, he pulled the emergency brake, causing the train to lurch to a stop.

"I exposed myself because I wanted the attention," Thompson told cops after his 2019 arrest, according to court papers. "I pulled the emergency brake on the 2 train. I did it to mess with transit and for the fun of it."

Thompson is currently facing multiple counts of assault, menacing and weapons possession for slashing the conductor last June. Prosecutors said they have video surveillance of Thompson at the scene.

The conductor is also set to testify, Flanders said.

Thompson's attorney, Laura Weiner, found it confusing that the only person testifying will be the conductor since the attack took place in a bustling Bronx train station and a train car filled with passengers.

"You will hear no testimony from any other person who was on the 6 train that day," she said. "You will hear no testimony from any other person who was on the 6 train platform on that day. You will hear no testimony from any other witness except the person who's accusing (him)."

"At the close of this trial you will have serious doubts about important parts of that story," she said.

During the opening arguments, Thompson sat quietly in a gray collared shirt, trying not to notice the growing group of MTA union members filling up the left side of the courtroom.

Drummond said the crystal-clear surveillance footage helped police find Thompson and is "compelling."

"It looks like him. It's him!" he said.

The trial began just hours after police released images of a man wanted for pummeling a 51-year-old transit worker standing on the southbound E train platform at the 71st Ave. subway station in Forest Hills, Queens, during an argument Wednesday. Cops were still looking for the assailant in that case.

"She's not the first. She's just the most recent," Drummond said of the slashed conductor. "We have dozens upon dozens of our brothers and sisters who've been slashed, spit, punched, assaulted, attempted rape."

"We absolutely, without question, right now have the most dangerous job in New York City," he added. "We don't carry guns. We don't have tasers. We're not allowed to have mace, and it seems to me we're prey for these monsters."

"All we're trying to do is get you home safely," he added.

Thompson's trial is expected to continue Monday.