Cardinal Jaime Sin ordered all Philippine Roman Catholic churches today to ring their bells each noon, starting Monday, as part of a campaign to accelerate investigation of opposition leader Benigno Aquino Jr.'s assassination.

In another development, opposition leader Salvador Laurel resigned his seat in the National Assembly, saying President Ferdinand Marcos "is bent on a policy of persecution and oppression that could engulf the whole nation in a tragic bloodbath. I will have no part in this sinister purpose of this notorious government."

Laurel heads the a coalition known as the United Democratic Opposition. Marcos' party dominates the assembly.

In a pastoral letter, Cardinal Sin urged Filipinos to stop work every day at noon and pray "for justice and peace in our nation" and to ask "the Holy Spirit to enlighten the minds, open the hearts and strengthen the wills of the members of the special commission investigating the assassination."

Sin is said by sources close to him to be disheartened by the lack of a credible explanation from the government of the mysterious circumstances surrounding Aquino's death.

A source close to Sin said the prelate ordered the bells rung 21 times each day because the figure marked "the end of democracy" when Marcos declared martial law on Sept. 21, 1972, and because it was on Aug. 21 that Aquino died--the day that Sin, referring to the public display of outrage, said "democracy came back to life again."

The ringing of bells and saying of prayers are to continue until the commission set up to investigate the murder has completed its work. A church source said it was the cardinal's way of "nagging them not to dillydally."

Action of the commission, headed by Chief Justice Enrique Fernando, was suspended Monday after two days of hearings. The Supreme Court is to rule on petitions seeking to disqualify the five members as being too close to the government.

Aquino, Marcos' political archrival, was killed at the Manila airport by what the government said was a lone assassin moments after his return from three years of self-imposed exile. The government has denied opposition charges of complicity.

Police Maj. Gen. Fidel Ramos said yesterday the situation here was tense as a boycott of schools and newspapers began to pick up. Students at 14 schools operated by the Catholic Church boycotted classes Monday and today.

About 3,000 students staged a peaceful downtown demonstration today, burning effigies of Presidents Reagan and Marcos.

Many columnists individually reported that a boycott of the three major dailies owned by Marcos' friends seemed to be succeeding. Newsboys have reported that their sales in the streets have slumped this week.

Yesterday, Marcos accused the opposition of obstructing the work of the commission, asking, "Is it because they the opposition know something which, if brought out in the investigation, they would not be able to handle?"

Marcos' statement is seen by observers as reflecting apprehension that the slowness in getting the commission underway is now creating doubts about whether Reagan will make his scheduled November visit.

A White House advance team inspecting facilities in Manila during the weekend indicated that Reagan's activities during the 24-hour stop would all be indoors.