The young Pakistani immigrant behind an abortive bomb plot wanted to back out of the plan in the days before his August 2004 arrest, he testified yesterday during his trial at federal court in Brooklyn.

As the terrorist plot took form and a dry run was scheduled, Shahawar Matin Siraj, 23, said he began to tell a co-conspirator why the plan to bomb the Herald Square subway station could not go forward. One reason he gave was that the explosives, which had not yet been obtained, would weigh too much for him to carry in a backpack.

Yesterday was Mr. Siraj’s first day on the witness stand. If convicted of charges of conspiring to blow up the subway station, he faces between five and 20 years in prison.

The defendant, formerly a clerk at an Islamic bookstore in Brooklyn, told of growing up in Karachi, Pakistan, where his lack of interest in Islam led to troubles in school. Instead of studying, he was interested in playing cricket and video games, he testified. In 1999, he emigrated to Queens.

“I wanted to give blood,” Mr. Siraj said, referring to his feelings after the September 11, 2001, terror attacks. “But I was suffering an allergy from working in the basement where there is rat powder and my mother wouldn’t let me.”

It was in Bay Ridge, Brooklyn, where Mr. Siraj worked at his uncle’s Islamic bookstore, that he gravitated towards radical Islam.

His legal team has based its defense around the argument that a police informant entrapped Mr. Siraj, by encouraging in him the anti-American views that led to the terror plot. Yesterday, Mr. Siraj suggested repeatedly that the police informant’s endless talk of jihad pushed him to suggest blowing up the subway station.

Mr. Siraj became acquainted with the informant from his visits to the bookstore in late 2003. Mr. Siraj sought the approval of the man, Osama Eldawoody, one of his lawyers, Khurrim Wahid, suggested.

“And when you told Mr. Eldawoody about the idea?” asked Mr. Wahid, referring to Mr. Siraj’s first mention to target Herald Square.

Mr. Siraj answered: “He was really happy. He said, ‘you’re a very smart brother….’ At the time I felt really good.’

As the two men prepared to survey the subway station, Mr. Siraj began to give his list of reasons to Mr. Eldawoody of why he could not be a part of the plan, he testified yesterday. To convince Mr. Eldawoody to suggest dropping the plan, Mr. Siraj testified that he made up a story about how security would be raised at the subway station following death threats he said President Bush had received.

Mr. Siraj testified that Mr. Eldawoody responded to his worries by asking him if he was backing out of the plan.

“I couldn’t say no to him,” Mr. Siraj testified.

Prosecutors disputed Mr. Siraj’s contention that his terrorist plot developed out of his relationship with Mr. Eldawoody. During the one hour of cross-examination he was permitted yesterday afternoon, an assistant U.S. attorney, Marshall Miller, questioned Mr. Siraj on statement after statement that he reportedly made in 2002 before he met Mr. Eldawoody.

“There is no way you praised Osama Bin Laden before you met Mr. Eldawoody?” the prosecutor, Mr. Miller, said. “That’s your testimony?”