WASHINGTON -- President Reagan, in almost six years in office, has approved at least 50 secret operations designed to shape events at home and abroad, administration and congressional sources say.

From Iran to Nicaragua and from Cambodia to Chad, U.S. agents have armed anti-Communist rebels, helped stage a successful revolution, manipulated elections, mounted propaganda campaigns and blocked supplies to leftist guerrillas.

At home the administration has attempted to influence press coverage of Nicaragua and El Salvador and has monitored Americans opposed to U.S. policies in Central America.

None of the programs has generated as much controversy as recent revelations of secret contacts with Iran amid allegations that the Reagan administration authorized arms shipments to that country to gain the release of Americans held by pro-Iranian groups in Lebanon.

But the Iran affair has drawn attention to other little- noticed covert action programs and may lead to new congressional probes. At least one of the programs -- the approval of a shipment of U.S.-made tanks to Guatemala by a Belgian company -- came at a time when Congress had outlawed military aid to Guatemala.

The proliferation of covert operations also has caused dissension within government ranks. John McMahon, the CIA's deputy chief, resigned in March. McMahon cited only "personal reasons" for his departure, but administration sources have said he left because he felt the administration turned too readily to covert programs. Sources said McMahon did not object to the administration's goals, but did fear that the United States could lose control of covert programs and that their exposure and subsequent investigation could damage the CIA's reputation.

While most American presidents in recent history have authorized covert actions, Reagan's secret operations are different.

Previous projects began and ended relatively quickly, were generally carefully monitored or were limited to specific goals. If exposed, they were terminated immediately, or exposure came after the fact.

With only two or three exceptions, however, Reagan's covert operations have been open-ended, continue even if exposed, and grow in size and scope. For example, the Nicaraguan contra aid program began as support for a small force of commandos whose stated goal was to intercept arms shipments to leftist guerrillas in El Salvador. The program now finances an army of more than 10,000 combatants.

The Reagan administration also has changed the way covert action programs are carried out. Not only is the CIA -- the agency traditionally charged with undercover foreign policy -- involved, but the White House has directed many of the actions itself, through the National Security Council. That appears to have been the case especially when officials wanted to skirt bureaucratic and congressional constraints, sources said.

The White House and the CIA declined to comment officially.

ADVANCING U.S. INTERESTS

But an administration official said the United States implements secret programs "not for sinister reasons" but to advance strategic American interests.

"The United States is not the only nation in the world that carries out secret operations. Most countries do, " the official said. "They are a necessary component of foreign policy."

Reagan's use of the CIA and NSC to conduct the 50 secret operations since he took office in January 1981 marks a return to the style of the 1960s and early '70s, when various administrations conducted some 900 large and medium-sized secret actions and thousands of smaller missions worldwide, the sources said.