FBI Director William H. Webster said yesterday that the bureau would not interfere with "lawful political dissent" despite new guidelines expanding its authority to infiltrate domestic political groups.

Testifying before the House Judiciary subcommittee on civil and constitutional rights, Webster said that the guidelines "allow us greater flexibility in the early stages of an investigation," but "they do not alter our clearly stated obligation to respect constitutional safeguards reflected especially in the First Amendment."

Webster said it was unlikely that mere advocacy of criminal activity--such as a speechmaker calling on his audience to refuse to register for the draft--would trigger any kind of investigation.

But he said he would not want to rule out that possibility, especially in situations where the statements made "suggest a serious and immediate prospect of harm."

Rep. Don Edwards (D-Calif.), the subcommittee chairman, said he still had misgivings in light of the ambiguity of the guidelines and "the possibility that some future FBI may place a different interpretation on them."

Issued last month by Attorney General William French Smith in place of the original guidelines promulgated in 1976 by then-Attorney General Edward H. Levi, the new rules:

* Allow the FBI to use informers and infiltration by undercover agents in the course of a preliminary inquiry before there is enough evidence to warrant a full investigation.

An "allegation or information indicating the possibility of criminal activity" is sufficient to touch off a preliminary inquiry.

A full investigation requires a "reasonable indication" that a federal crime has been or is about to be committed.

* Authorize the FBI to conduct an investigation based on advocacy of criminal activity, especially violent criminal activity, "unless it is apparent from the circumstances or the context in which the statements are made, that there is no prospect of harm."

The Levi guidelines were silent on the advocacy issue.