This is the height of Venice's tourist season. The cafes in San Marco Square are crowded and gondolas filled with American and Japanese tourists glide up the Grand Canal.

But this week, this lagoon city famous for its canals, monuments, churches, and omnipresent hungry pigeons will have some less traditional guests. The additions to Venice's usual tourist population will include seven heads of government and their entourages, an estimated 1,300 journalists, and 5,000 soldiers and police sent from other parts of Italy to protect them.

Italian police, carabinieri, customs guards and soldiers patrol Venice's ancient piazzas and streets with their black, snub-nosed submachine guns adding a forbidding touch to the usual Venetian circus atmosphere.

A few months ago, Italian terrorists murdered a high-ranking Venetian policeman and warned in an accompanying statement that they would make trouble in June at two summits -- this weekend's international summit and last week's Common Market conclave.

But almost nobody expects any violence.

"What is all this nonsence," muttered one Venetian newspaper vendor whose kiosk was isolated from prospective customers by a police cordon in front of the 12th century Doges' Palace where a lunch will be held on Sunday.

While the Venetians may affect nonchalance at the arrival of world leaders like President Carter, British Prime Minister Margaret Thatcher, West German Chancellor Helmut Schmidt and French President Valery Giscard d'Estaing, the Italian government is clearly taking no chances.

Armed men are stationed along the Grand Canal's landings. Others flank the doorways of the luxury hotels where the European, Canadian and Japanese delegations are staying. Still others block access to the motor launches at the glamorous Cipriani Hotel where President and Mrs. Carter and Amy will stay.

On the Grand Canal near the late Peggy Guggenheim's famous museum, several motorboats fly the American flag. The boats have been rented by the U.S. government for $35 an hour and are docked outside a low waterfront building. The Venice headquarters of Wake Forest University has been temporarily taken over by the U.S. government and transformed into a communications center.

The U.S. government has also set up a press center at the luxurious Excelsior Hotel on the beach on the Venice Lido. Three hundred American reporters, who with the Japanese form the largest contingent of journalists covering the summit, are staying at the Excelsior and the Des Bains Hotel, which was the setting for the film version of Thomas Mann's novella, "Death in Venice."

The hotels have jacked up their prices to $245 a day for a single room with a bath. One night during last week's European summit, a gondolier wanted $35 to ferry an American reporter directly across the Grand Canal.

The Italian government has spent more than $6 million preparing for the two summits. Most of the money was spent to build a huge, ultramodern press center on the island of San Giorgio Maggiore where the conference center in which the world leaders will meet is located.

International phone and telex lines, transportation to and from the island for accredited participants, meals and generous refreshments have all been provided with impressive efficiency. Water traffic has been coordinated so that the lagoon will be cleared when the heads of state travel to and from the island. A complicated system of identification passes with different colors for different countries and diplomatic ranks also has been worked out.

However, the identification system may not be perfect.

One day this week, a boatload of what appeared to be U.S. Secret Service agents tried to approach the island and were challenged by Italian policemen. Perhaps frustrated by language difficulties, the Italian police reportedly sprayed the water with machinegun bullets.

The incident, which was reported by an employe of a U.S. wire service but not in the Italian press, betrays just how nervous the police are.

A high-ranking diplomat at the Foreign Ministry in Rome laughed when he was told that Venice was chosen as the site for the summit because it was easy to protect.

"We Italians are not so rational in our planning," he said. "The only reason Venice was chosen is because it is an exquisitely beautiful place."