A senior Panamanian diplomat broke with his country's military-dominated government yesterday, saying that he was resigning because he could no longer serve a regime stained by "corruption, illegitimacy and dishonesty."

Lawrence Chewning Fabrega, minister counselor at Panama's mission to the Organization of American States (OAS) here, said at a news conference that his decision to resign was triggered

by a speech he was ordered to give at an OAS ceremony last Wednesday.

In the speech, Chewning had accused the Reagan administration of trying to overthrow the Panamanian government and waging a campaign of "moral and economic aggression" against Panama.

"These remarks were not written by me, nor did they represent the way I feel or think," Chewning said yesterday. He said the remarks were prepared by Panama's ambassador to the OAS, Roberto Leyton, and the Panamanian Foreign Ministry.

The resignation is the latest development in the controversy over Panamanian strongman and de facto ruler Gen. Manuel Antonio Noriega, who was indicted last week by two federal grand juries in Florida on drug-related charges and

was accused by former associates before a congressional panel of engaging in an array of corrupt activities.

Sen. Alan Cranston (D-Calif.), a member of the Senate Select Committee on Intelligence, yesterday said he has asked the committee to look into charges made in testimony before a Senate Foreign Relations subcommittee this week that the Central Intelligence Agency had paid Noriega for information.

Chewning, speaking through an interpreter at OAS headquarters, said: "My conscience, my dignity and my deep religious beliefs prevent me from continuing to be used as an instrument of a government characterized by corruption, illegitimacy and dishonesty."

Leyton, Panama's OAS ambassador, was not available for comment. Gabriel Lewis, a former Panamanian ambassador who has been lobbying here against Noriega's rule, said Chewning's resignation proves there are "worthy individuals" in the Panamanian government who refuse to continue supporting Noriega.

At his news conference, Chewning said he thinks that the United States will comply with its commitments.

Chewning said he is not seeking political asylum in the United States and is confident he will not be the target of reprisals by the Panamanian government.

His visa allowed him to remain here only as long as he served as a diplomat. He said he has about a month to decide what his next step will be.

Chewning, 46, a veteran diplomat, served in the OAS post for a year and a half.