MACHIAS, Maine — A 16-year-old girl testified Monday that a Calais man on trial for eight sex crimes against three young girls forced her six years ago to perform oral sex on him while she was attending a party at his home.

The incident took place in the bedroom of Clifford Thornton Jr. in the spring of 2007-08, recounted the teen, who was 10 at the time.

Her testimony came during the first day of the Washington County Superior Court trial of Thornton, who is accused of one count of gross sexual assault and seven counts of unlawful sexual contact involving three girls who were under age 12 or 14 at the time. Thornton was 50 at the time of his arrest in August 2011 and has remained free on bond. The trial is expected to continue through the week.

The alleged offenses, which purportedly all took place in Thornton’s home or on his property, date back to the mid-1990s, with the most recent offense in 2008.

Adults and children were attending the party, according to the teen witness, who said she had been forbidden to go outside and play with other children because she was dressed up for the occasion. Thornton suggested she could play in his bedroom, where he had a computer and an Xbox video game, she testified.

When the two were alone in the room, he closed the door and showed her a picture on the computer of a nude girl she recognized, the teen testified. The photograph had been taken in the same bedroom.

“He began to unbutton my shirt,” she testified. He pulled up her blouse, partly exposing her breasts and touching her there, and he also unbuttoned her pants, she said. Whenever she protested, he told her “shush.”

Thornton lowered his trousers and forced her to kneel in front of him and perform oral sex on him, she testified.

“I was just baffled at what was happening,” she said.

When the episode came to a close, a relative called for her from the kitchen, she quickly composed herself, and she and Thornton left the room together.

She did not tell anyone about the incident because she did not want to get into trouble, the teen testified. She eventually told a relative in 2011, and that person relayed the information to the teen’s mother, who notified Calais police. The teen said she wrote an account of the incident in a notebook in order to bring it to a meeting with a police officer.

Under questioning by First Assistant District Attorney Paul Cavanaugh, the teen disclosed that she entered counseling because she deliberately cut herself. The cutting incidents followed a series of health classes in which she and other students were taught about sexual assault, she said.

Under cross-examination by Bangor defense attorney Stephen Smith, the witness admitted that there were some discrepancies between her trial testimony and the information she initially supplied to police. For example, she told Cavanaugh that the counseling was not related to drug or alcohol abuse, but in the journal account she gave to police she referred to using pills, other drugs and alcohol.

She also conceded differing testimony in response to defense and prosecution questions. For example, she told Cavanaugh that Thornton had not touched her anywhere else, but she told Smith that Thornton also had touched her genital area.

The jury of six men and six women began listening to testimony after being impaneled in the morning and then traveling by bus to Calais to visit the Thornton home and property where the crimes allegedly occurred. Attorneys for both sides and Justice E. Allen Hunter also traveled to Calais to accompany the jury.

Monday’s witness offered testimony related to the charge of gross sexual assault. Six counts of unlawful sexual contact — any touching of the genitals, even when the victim is clothed — pertained to a second victim, and one count to a third.

In the one charge of unlawful sexual contact against one victim, Cavanaugh said in his opening statement that Thornton gave her a ride on a snowmobile. As she was seated in front of him, he grabbed her crotch through her snow suit and thrust himself against her, said Cavanaugh.

In the six charges of unlawful sexual contact related to another victim, Cavanaugh said Thornton showed pornographic images to her and had her name body parts as a prelude.

At least two of the victims later began experiencing mental or emotional problems and used drugs, said Cavanaugh.

Defense attorney Smith began his opening statement by showing the jurors a diagram of Thornton’s home and property. He noted the “very tight quarters this family lives in.” He also said the family was very “sociable” and frequently had guests in their home, including friends of their children.

Thornton’s wife, Lisa, operates a licensed day care facility out of a portion of the home, said Smith. The victims were not clients of the day care business, and the crimes did not take place in the day care portion of the home, according to the prosecution.

Thornton has continued to live at home while he has been free on bond pending trial.

Authorities reported the offenses against Thornton to state regulators who oversee day care facilities, and state officials conducted an inspection of the day care business but allowed it to remain open, said Cavanaugh and Sgt. Chris Donahue of the Calais Police Department.