President Reagan was confronted today by derisive protests from thousands of laid-off steelworkers as he carried a message of economic hope in the computer age into the nation's troubled industrial heartland.

A rain-soaked crowd of about 4,000 blue-collar workers, chanting "Reagan, Reagan, he's no good, send him back to Hollywood" and carrying placards such as "Jobs Not Bombs," assembled outside the Pittsburgh Hilton, where Reagan delivered a speech on job retraining.

But the president's limousine was diverted into an underground garage to avoid a direct confrontation with the demonstrators, who booed and jeered as his motorcade approached. Some carried banners of the United Steel Workers union and others held signs, "Feed the Hungry, Not the Pentagon," and "This is not Reagan country, 17.2 percent unemployed in western Pennsylvania."

The protests were not the only reminder of economic hardship on a day in which Reagan sought to highlight economic recovery and what he described as a "basic transition" from smokestack industries to high technology.

After speaking to a class of 120 blue-collar workers being retrained for computer repair and maintenance, one trainee, Ron Bricker, unexpectedly called out to Reagan and offered the president his resume.

"I've been looking for a job for a year. And I can't find one," he said as his colleagues voiced their approval.

Unemployment in Pittsburgh, which topped 17 percent in January, now is 16.2 percent. Reagan's trip here was designed to address his larger political problem of disenchantment among the blue-collar workers, who were key to his 1980 victory.

The steel towns of the industrial states often were used as a backdrop for Reagan's appeal to those voters in the 1980 campaign, but he received a muted and sometimes hostile reception as he returned to Pittsburgh for the first time as president.

As his motorcade sped through the city, one shop owner held up a sign that read, "Thanks Ron, business is off 50 percent." The rally, protesting Reagan's policies, also included a skit by the "Not Quite Ready For Unemployment Players."

The president, who spoke to the computer trainees at the Control Data Institute and later addressed a conference on "dislocated workers," took note of the political overtones in his visit.

"I come not only as a speaker but as a possible victim," he told the conference on dislocated workers. "I assume there are a number of Democrats who would just love to dislocate me."

But he urged businessmen to "help those frustrated steelworkers, some of whom are across the street venting their confusion and anger as we meet."

Reagan played on some of that frustration and anger in 1980, when he blamed President Carter for unemployment, but absent from his remarks today was any acknowledgment that he had failed to deliver on his campaign promise to reduce joblessness.

Rather, he said, the country "is going through a basic technological transition" to the computer age "and that, along with the stagnation of recent years, has created a large-scale unemployment problem."

He said that "structural unemployment" was caused by "deep and lasting changes in science, technology, competitiveness and skills." Reagan again used the illustration of help-wanted advertising in newspapers to make his point that jobs are going for the asking because workers lack skills.

The president's suggestion that retraining is an answer to joblessness was greeted skeptically by one trainee, Ron Cassidy, 29, who was laid off last June from the open-hearth operation at the now-idled U.S. Steel Homestead mill.

"We are part of the lucky few that have gotten in," he said. "A lot of older guys feel they are 40, what's the use of retraining? If you worked at a mill 20 or 25 years, it is hard to change."