Indian American Dharun Ravi urged other students to spy on his roommate's gay date, a New Jersey court was told as defence suggested a key witness agreed to testify to have criminal charges against her dropped.

Ravi is on trial for spying on his roommate Tyler Clementi during a date with another man. Days later Clementi jumped to his death from the George Washington Bridge.

Alissa Agarwal, a Rutgers New Jersey State university student who was friends with Ravi, and Molly Wei, who was originally charged along with Ravi, testified for the prosecution Monday.

Agarwal told the court that she received a tweet from Ravi Sep 19 that read, "roommate asked for room for the night. I went to molly's room and turned on my webcam and he was making out with a dude. Yay."

She said she received another tweet from Ravi on Sep 21 supposedly alerting his friends that Clementi was going to have another date and suggesting they watch by contacting his webcam.

"Anyone with ichat, I dare you to ichat me from the hours of 930 and midnight. Yes, it is happening again," Ravi tweeted.

Agarwal said that on the evening of Sep 21 Ravi explained to her in the presence of other students that his webcam was set on auto accept "in order for multiple people to be able to view Tyler's side of the room."

He also demonstrated for her by opening up her iChat programme and it showed Clementi's side of the dorm room.

Wei told the court that after she was initially interviewed by police, she contacted them again on Sep 27 because she wanted to give them more information.

Ravi's lawyer Steven Altman asked Wei that when she voluntarily went to the police a second time, "Did you think you had committed a crime?"

"I personally felt like I had did something wrong, but I didn't understand the law...I didn't understand what I did to get these charges," Wei replied.

"Is it true you didn't want to go to jail?" Altman asked.

"Yes," she replied.