U.S. Ambassador Deane R. Hinton, departing from the Reagan administration's "quiet diplomacy" on human rights abuses in Latin America, warned Salvadorans in sharp language today that unpunished murders and kidnapings tied to "some elements of the security forces" are endangering U.S. support for their government.

Hinton's warning came in a speech to the U.S.-Salvadoran Chamber of Commerce. U.S. officials said the speech had been cleared by Washington, and it seemed designed as a particular expression of concern about the arrest of eight leftist activists and the abductions of others during the past two weeks.

"Neither internal confidence nor external support can long survive here in the absence of an effective system of criminal justice," Hinton said, adding later: "The 'mafia' must be stopped. Your survival depends on it. The gorillas of this mafia, every bit as much as the guerrillas in the provinces of Morazan and Chalatenango, are destroying El Salvador."

The ambassador's reference to external support clearly meant U.S. military and economic aid, which he reminded his audience amounts to more than $230 million this year and includes about 50 U.S. military advisers for the Salvadoran Army in its fight against leftist guerrillas backed by Nicaragua.

Under U.S. law, the Reagan administration must certify to Congress every six months that El Salvador is making progress in human rights as a condition for continuing the aid. The next certification is due in January.

Against this background, the arrests and abductions could turn into ammunition for congressional critics of the administration's determination to aid El Salvador's government against the insurgents. They come on top of the government's inability or unwillingness to resolve two cases: the murders of two American labor officials machine-gunned along with a Salvadoran colleague in a hotel coffee shop in San Salvador on Jan. 3, 1981, and of four U.S. churchwomen killed in 1980 on the way from the country's main airport.

"El Salvador must make substantial progress in bringing the murderers of our citizens, including those who ordered the murders, to justice; in advancing human rights; and in controlling the abuses of some elements of the security forces," Hinton said. "If not, the United States, in spite of our other interests, in spite of our commitment in the struggle against communism, could be forced to deny assistance to El Salvador."

Hinton, speaking in Spanish from a prepared text, told his mostly Salvadoran audience that the U.S. Embassy has reports of 68 persons "murdered in El Salvador under circumstances which are familiar to everyone here."

"Every day we receive new reports of disappearances under tragic circumstances," he added. "American citizens in El Salvador have been among the murdered, among the 'disappeared.' Isit any wonder that much of the world is predisposed to believe the worst of a system which almost never brings to justice either those who perpetrate these acts or those who order them?"

In about 50 mostly hostile questions forwarded to Hinton after his speech, a number of Salvadoran businessmen challenged Hinton to say by what right he was lecturing El Salvador on law and order or, in the words of one, "blackmailing" the Salvadoran leadership with threats of a U.S. aid cutoff.

Hinton took care not to attack specifically the government of President Alvaro Magana or blame it for what he called wanton murder by extremists of the left and right. But his remarks clearly chided the government for failing to live up to commitments on human rights.

"It is not enough that El Salvador's constitution and laws protect individual rights, that El Salvador subscribes to a long list of international human rights conventions," he said. "The reality must change to more closely match the ideal."

Hinton's choice of a forum was seen as significant. The Chamber of Commerce membership includes some of El Salvador's leading businessmen from families often identified with the right-wing politics whose extremist advocates in the Army and security forces are widely believed to be involved in some of the abuses.

Evidently in reference to this, Hinton called for reform of the judicial system but warned that "all will come to naught unless the will exists in this country to make it happen, unless the will exists to punish those who are responsible -- regardless of their station in life."

What in another country could be a routine, disagreeable expression of U.S. policy amounted here to a serious warning. The United States plays a crucial role in El Salvador's political life. With military aid alone amounting to $125 million in the last 20 months, Washington also has decisive influence in the three-year-old war against an estimated 4,000 leftist guerrillas -- which the Reagan administration says are aided by Nicaragua's Sandinista revolutionaries.

Hinton was thrust into an especially uncomfortable position by the most recent series of arrests and abductions. The families of five well-known leftist politicians first raised the alarm about them having been picked up, some by heavily armed men in civilian clothes.

Queried last week by an American correspondent, Hinton expressed dismay and said he was fairly sure the abduction had not been carried out by the government. But early this week the Defense Ministry announced it was holding eight leftists, accusing them of links with the Democratic Revolutionary Front. This group, whose leaders have fled the country, is the political arm of the umbrella guerrilla organization, the Farabundo Marti National Liberation Front.

At the same time, El Salvador's left-leaning Human Rights Commission reported complaints from families testifying that 19 persons in all have been abducted since Oct. 10, including some of those acknowledged by the Defense Ministry and a number of others whose fate remains unknown.