It was 10:30 in the evening and Calle Lavalle was full of strollers enjoying the warm spring weather. Restaurants and theaters on this downtown street were doing a good business.
Suddenly from a side street came the wail of a siren. A police van, loaded with armed agents, nudged its way down Lavalle, scattering pedestrians.
The van pulled up to a brand new fast-food franchise - Pizza Hut - and the agents dashed into the restaurant.
Before long, they had checked the ID cards of diners and taken away three women who were unable to produce their cards. The women probably were released later, but a check with the police failed to disclose information on their whereabouts or identities.
Why had Pizza Hut been singled out from among 27 restaurants in four blocks of Lavalle?
''Just a routine check,'' said one agent, holding his loaded revolver.
But the incident reminded many of the way police and Army units have been operating here during the past 71/2 years of military rule. Raids on restaurants , theaters, meeting houses, and homes were commonplace, particularly in the late 1970s. There is less of that today, but it still persists.
Some diners wondered if the raid was a shakedown. Had the Pizza Hut owners failed to grease the palm of some local police chief?
''You know,'' said one diner, ''This is still the country of police and military mafias. They can get away with anything.''
But the raid clearly surprised some Pizza Hut diners. ''I thought these raids were a thing of the past,'' said one. ''After all, the state of siege has been lifted.''
A spokesman at the Ministry of Interior said,''This is a matter for the police,'' after he was queried about the Pizza Hut incident. When it was pointed out that the police are nominally under control of the Minister of Interior, the spokesman, Brig. Gen. Llamil Reston, said, ''Yes, but police are free to do what they please.''
That is the basic problem regarding the Argentine security forces, which include the military and the police, as far as most Argentines are concerned.
Pizza Hut was not the only restaurant raided recently. Eating spots in the chic Barrio Norte area of town also were raided. None of the owners or managers of the establishments wants to be identified, nor do they want their restaurants mentioned. They say police came looking for ID cards. At other times, police are reported to have headed for the kitchen, alleged there were infractions of sanitary codes, and then offered to overlook the infractions for bribes.
''We get dozens of these reports each week,'' says a Buenos Aires newsman. ''It is just the police flexing their muscles and showing their authority. Some police may worry that after Dec. 10 they won't be able to get away with such behavior.''
Dec. 10 is the day that civilian President-elect Raul Alfonsin assumes office , ending the military rule under which the police operated with virtual impunity.
But longlasting police and military practices will be hard to break. Dr. Alfonsin knows this. So far he has said little about how he plans to deal with the police. He may want to leave the problem to newly elected local officials.
However, Alfonsin has promised to restrain the military. His Radical Party has promised to do the same with the local police.
The President-elect says he will force the Army, Navy, and Air Force to accept a civilian commander in chief. He also says he will reach way down in the ranks of generals for an officer to become head of the Army. One name mentioned for the post is Brig. Gen Ricardo Roberto Flouret. If he is chosen, 50 generals will be forced into retirement, since no officer on active duty can be higher in rank than the chief of the Army. Such a move would in some measure undercut the military.
So would other Alfonsin plans. He has announced his intention to slash the military budget and to end the draft.
Some of his supporters urge him to go further. Some would like a vendetta against the military and the police involved in the so-called ''dirty war'' of the late 1970s, in which thousands of Argentines were killed and other thousands simply disappeared. The military said its actions in this era were part of an effort to do away with subversion.
To protect itself from a vendetta, the military government promulgated an amnesty law last September exempting any military man or any policeman from prosecution for alleged crimes during the past 71/2 years. That law is already being challenged in the courts and Alfonsin promises to push for its annulment in the new Congress.
Still, it may prove difficult to bring to trial military and police charged with crimes during the dirty war. In the first place, specific evidence will be hard to obtain against individual soldiers or policemen. And under Argentine law , prosecutors must produce bodies in the cases of those said to have been killed by the military.
''But you have to realize there are no bodies,'' one general said recently.
In addition, it may also be very hard to bring specific individuals to trial, since a person cannot legally be brought to trial for a crime once he received amnesty. Lawyers, however, are looking for loopholes in this legal tradition.
Incidents like the one at Pizza Hut are clearly not as serious as those in which people were killed or abducted and never hear from again - assuming that the three women picked up that evening were released. Temporary jailings for failing to have ID cards are common.
But that incident is symptomatic of the freewheeling nature of police and military activity that still prevails. But there is hope among may of those close to Alfonsin that the new government will strongly clamp down freewheeling police practices and see too it that they do no arise again.
Meanwhile, business is back to normal at Pizza Hut - and the whole incident ''seems a bit like a bad dream,'' in the words of one of the waiters.