The suspected assailant was taken into custody by UCLA police, who sealed off Young Hall, home of the school's chemistry department. Police said they don't know of any motive for the attack.

The victim, whose name was withheld, was taken to Ronald Reagan UCLA Medical Center, where she apparently was undergoing surgery.

Witnesses said they saw a woman staggering out of the sixth-floor Young Hall lab with a teacher's assistant applying pressure to her bloody neck moments after the attack, which was reported at 12:21 p.m.

Law enforcement sources said some type of verbal altercation occurred just before the attack, but the relationship, if any, between the assailant and the victim was unclear.

One student who was inside the lab when the attack occurred shortly after noon told The Times that he looked up as the assailant appeared to repeatedly punch the victim. Then the man calmly turned and walked away as the victim lay bleeding profusely.

Students in the UCLA science lab described the horrifying moments this afternoon when a male classmate slashed the throat of a female student, who was critically injured.

"We have asked students to stay away from Young Hall," said UCLA police spokeswoman Nancy Greenstein.

The attack apparently took place between class sessions in an organic chemistry lab on the top floor of Young Hall.

The undergraduate level lab, known as 30CL, enrolls about a dozen students and is usually led by teaching assistants. It is part of a class for about 60 students that is overseen by lecturer Alfred Bacher, according to department officials.

Bacher, who joined the UCLA faculty in 2001 after work as a postdoctoral fellow there, could not be reached for comment.

By at least one account of a chemistry department employee who asked not to be identified, the assailant turned himself in to a campus employee in a third-floor reception and mail room area in the building.

UCLA chemistry professor Robin Garrell, who is chairwoman of the UCLA campuswide Faculty Senate, was leaving her fourth-floor office in Young Hall when she saw emergency crews wheeling out the victim on a stretcher on the ground floor.

Students and faculty “are obviously very shaken” by the incident, Garrell said. “It’s very shocking.”

Psychological counselors were on the scene to help students cope with the situation, she said.

Two hours later, traumatized students who witnessed the attack, still in their lab coats, lined the sixth floor, waiting to be interviewed by detectives. Many of their personal possessions, including books and keys to their cars and residences, remained behind in the lab room that had become a crime scene.

[Updated at 4:45 p.m.: UCLA campus spokeswoman Carol Stogsdill said she had no information about the alleged assailant's past behavioral record and had not heard of any reports of previous trouble in the class or between the two students. She said both students were 20-year-old seniors and that their identities were being withheld by police.

Chemistry department vice chairman Peter Felker said he did not know any of the students in the class and that the department had not received any reports of trouble in the lab or complaints about the alleged assailant’s past behavior. "Nothing that I’m aware of,” he said.

UCLA biochemistry professor Sabeeha Merchant was close to the crime scene soon after the stabbing and praised the other students and faculty who came to the aid of the victim and alerted police. "They were responsible and calm and really helped,” she said.

Merchant and others described the mood in Young Hall as stunned. “People are shocked that something like that would happen. But because we are at the university doesn’t mean we are immune from what goes on in the rest of the world," she said. "This could happen in a restaurant or a shopping mall.”]

—Spencer Weiner and Anthony Pesce at UCLA, Andrew Blankstein and Larry Gordon in Los Angeles

Click here for photos from the scene

Photos: UCLA students wait to be interviewed, top, by campus police, right, after a male classmate slashed the throat of a female student at a campus science lab.

Credit: Spencer Weiner / Los Angeles Times