The racially charged case was moved to Los Angeles for trial due to extensive publicity in the Bay Area.
Mehserle testified Thursday that when he arrived with his partner to the train station in response to a possible fight, that he could hear yelling and screaming from the platform above.
"I remember it being real loud," he said. "I didn't know if officers were involved in the fight or the crowd had turned on them. It didn't sound good."
Testifying in his own defense, the 28-year-old described for jurors his struggle with Grant, saying he was standing over the man trying to handcuff him. Mehserle said Grant was on his knees then fell to the ground.
"My main focus was his hands," Mehserle said. "I was trying to get him handcuffed," but "his right hand was underneath him."
At that point, Mehserle saw Grant reach into his right pocket. He told jurors he had been involved in past cases when people carried small handguns in their pocket.
Mehserle said he told Grant he was going to Taser him, after which Mehserle stood up and backed away from Grant and reached for the stun gun. He said he didn't realize he was grabbing his pistol instead of the Taser.
Mehserle broke down in tears as he told jurors he mistakenly pulled out his gun instead of his Taser and shot Oscar Grant. Mehserle said he heard a "pop" and thought his Taser had malfunctioned.
For Grant's mother, the tearfully testimony was too much. She walked out as Mehserle sobbed on the stand.
"It's been almost a year and a half, and he hasn't offered any apologies. He hasn't said it was a mistake and now he's sitting on the stand and crying. I've been crying every day," said Wanda Johnson, Oscar Grant's mother.
There was another dramatic outburst in court when a Grant supporter was arrested after he yelled out at Mehserle to "Save those tears."
Friday's emotional testimony was the first time Mehserle has talked publicly about the Oakland shooting on New Year's Day last year.
Until now, cell-phone video and camera video of the shooting was all that was known about the fatal shooting.
According to Mehserle his Taser was the only option that crossed his mind. He said the thought of using his gun "never entered my head."
The defense calls it a tragic accident, but Alameda County prosecutors call it murder, and claim Mehserle used his handgun because officers were losing control of the situation on the train platform.
Grant's family says the former officer knew exactly what he was doing.
"He knew the difference between his gun and his Taser," said Wanda Johnson. "He had it pointed at Oscar, you see that in the videos."
"He's now pleading with the jury, 'Help me, help me.' We're not feeling that," said Bobby Johnson, Oscar Grant's uncle.
Mehserle has pleaded not guilty to murdering Grant.
The case was moved to Los Angeles from Alameda County due to racial tensions and intense media coverage. The defense is expected to rest its case next week.
The Associated Press contributed to this report.