Ever since the hijacking of a TWA flight from Athens last summer, Greece's Socialist government has been fighting an uphill battle against charges from the United States and the conservative opposition that it is soft on terrorism.

Now, in the course of a few days, the issue has been raised again with a vengeance, not only at the airport, but in the streets of the capital.

First, there was the bomb explosion aboard another TWA flight traveling the now notorious Cairo-Athens-Rome route as it approached the airport here last week. It is still not clear where or when the bomb was planted, but the one suspect, having gotten off the plane on its earlier stop here, was able to depart unmolested for Beirut a few hours later.

Then on Tuesday a mysterious terrorist group murdered a 79-year-old Greek steel magnate in broad daylight in a fashionable downtown Athens shopping district.

While Athens can claim bad luck in the departure of the TWA bombing suspect, and U.S. officials here are not criticizing the way the police performed, the murder of industrialist Demitris Angelopolous has brought on a storm of controversy about the way the Socialists run the police and security forces.

The November 17 group that announced it had carried out the killing first appeared 11 years ago with the assassination of the CIA station chief here. Since then, it has killed or attempted to kill at least two other Americans, several former police officials from the military junta that ruled Greece from 1967 to 1974, conservative newspaper editors and riot police.

Yet little is known about its true aims and nothing is known about its members. Not one has ever been caught.

Only the day before the Angelopolous murder, conservative opposition leader Constantinos Mitsotakis of the New Democracy Party said this record "is not logical" and claimed that the police are in a state of "chaos." Yesterday, he went further and accused Socialist Prime Minister Andreas Papandreou of "guilt by association" in the killing.

The opposition repeatedly has criticized the morale and efficiency of the police force. Officers are said to be constantly on the defensive as Socialists accuse them of "fascism" when they try to do their jobs, and the government has been denounced for packing police ranks with its partisans.

In an interview this morning, Secretary General of Public Order Costas D. Tsimas, a political appointee in charge of the police, flatly called Mitsotakis "a liar."

Tsimas is at the center of the storm. Conservative opponents point out that he was a founding member of the Socialists' armed underground organization, PAK, during the junta years. Mitsotakis suggested yesterday that PAK and November 17 formed some sort of secret relationship then and called it "comic" that a man of Tsimas' background should be in charge of hunting for terrorists.

In his interview today, Tsimas testily answered these questions. He said he was proud to be a member of PAK, and acknowledged that he "had contacts with other liberation movements during that period."

But he said conservative claims that he was trained by Palestinians in Syria are "a lie," adding, "I have never been trained by any Middle Eastern force."

"My activities as a revolutionary stopped the moment the junta fell," Tsimas declared. He added that November 17 first appeared under a conservative government and that it had not existed under the junta.

"I used to know all the organizations that were against the junta. Where were they November 17 then?" Tsimas asked.

In a twist typical of the political confusion that surrounds the group, he suggested that it may not be leftist at all, despite the tone of its manifestos and statements.

The Greek Communists have claimed that November 17 is a group of agents provocateurs, working with conservatives to provoke repression. Tsimas did not go that far, but said that whatever the group's stated aims, "they are pushing the government to be more strict with the law."

Western diplomats as well as Tsimas point out that November 17 is difficult to combat because its attacks are sporadic and unpredictable. There have been 10 killings since 1975.

"Once a year they go out and execute somebody," Tsimas said. "Don't get this wrong, but the bad thing about November 17 is that they are not active enough."

Tsimas suggested that with no clear pattern to analyze and a membership apparently so small it cannot be penetrated, there is little he can do.

But he insisted that the Socialist reorganization of the 40,000 police in the last few years has been a success, and that there is no softness against terrorism. At least one Palestinian terrorist and an Armenian are still in jail here, according to police officials.

As for Greece's release of an accomplice in last summer's TWA hijacking, Tsimas said, that was done to save the lives of the plane's passengers. "The people who travel committed no crime that they should die," he said.