Enforcement of the nation's antitrust laws is "alive, well and thriving," and critics who assert otherwise "have wrongly interpreted a change in enforcement patterns as a failure to enforce the laws," the new head of the Justice Department's antitrust division said yesterday.

Assistant Attorney General Charles F. Rule, in his first meeting with reporters since being confirmed by the Senate in July, said that "particularly in an era of increasing international competition, we can no longer afford to attack American companies that are using innovative techniques to keep costs and prices down."

In an effort to "begin to put to rest the argument the law is not being enforced," Rule cited statistics showing an increase in the number of criminal antitrust cases being brought by the department.

He said 146 grand juries are currently investigating allegations of antitrust violations, including 35 probes of government procurement bid rigging -- "almost three times the 50 that were pending when we arrived in 1981 and, more important, a record for the antitrust division."

Although the number of civil suits filed by the antitrust division has declined during the Reagan administration, Rule said, the department has brought an average of 77 criminal cases a year during the Reagan administration, compared to 37 a year during the Carter administration.

"My number one priority as head of the antitrust division is simple: criminal cases and more criminal cases," he said.

Rule said the department is exploring the possibility of using the antitrust laws to attack organized crime figures that become involved in legitimate businesses and "often view price fixing or bid rigging as the normal way of doing business ... snuffing out competition wherever they go."

Rule, 32, a University of Chicago law school graduate who has worked at the Justice Department since 1982, had been acting assistant attorney general since Douglas H. Ginsburg was named to the federal appeals court here.

Critics have alleged that the antitrust division under the Reagan administration has all but abandoned broad areas of antitrust enforcement, approving all but the most extreme mergers and focusing almost exclusively on bid-rigging and price-fixing cases.

"The administration has done part of the job adequately but most of the job very poorly," said Sen. Howard Metzenbaum (D-Ohio), chairman of the Senate Judiciary antitrust subcommittee.

At a time when the number of mergers has soared, the department "has allowed one massive merger after another, and in the long run consumers are going to pay in higher prices," Metzenbaum said. "Antitrust may be alive, but it is not well."

The administration sought to block nine of 868 mergers for which companies sought approval in fiscal year 1979, but only four of 1,949 mergers in 1986.

"It's the most lenient antitrust policy we've ever seen," said Georgetown University Law Center Dean Robert Pitofsky, an antitrust expert who was a federal trade commissioner from 1978 to 1981.

"They haven't brought a single vertical or conglomerate merger case in seven years," and "they almost always find a way not to sue" to prevent horizontal mergers -- those involving companies in the same industry, Pitofsky said.

Rule said the department's 1984 merger guidelines "ensure that we challenge only those mergers that present a real threat to consumers."

Rule said that a 39 percent decrease in the number of antitrust division employes, from 939 in fiscal year 1979 to 565 in 1986, has not hurt enforcement because the division had become "much more effective in getting more for the taxpayer's dollar."

Asked to comment on the Supreme Court nomination of Judge Robert H. Bork, who made his reputation as an antitrust scholar, Rule said that Bork "is mainstream antitrust now," with his views that consumer welfare should be the sole goal of the antitrust laws and that certain practices, such as vertical and conglomerate mergers and retail price maintenance agreements, do not violate the law.

Pitofsky, who is advising the Senate Judiciary Committee on Bork's antitrust views, termed as "nonsense" Rule's assertion that Bork's position now represents the mainstream. "Even among conservative critics in antitrust, Bork is among the right wing of that group," Pitofsky said.