VENICE, Italy (AP) _ Under the weight of extraordinary security for the seven-nation economic summit, parts of Venice in the height of the tourist season have taken on the appearance of a city under siege.

With major waterways blocked off, essential services have been reduced to a minimum or suspended altogether in central areas of the city.

To placate unhappy tourists who have to walk or take long detours to major attractions, city officials have proposed substitute sightseeing routes for visitors to ''explore hidden treasures'' off the beaten track.

Gondolas, garbage-collecting barges, supply ships and water buses and taxis were banned from long stretches of canals near the island of San Giorgio - the summit site - and leading to hotels where the presidents and prime ministers are staying.

Also banned were water hearses, funeral barges, prompting Rome's Il Messasggero to declare that Mayor Nereo Naroni's order means, in effect, ''don't die during the summit.''

Some hotels, including the Metropole, a first-class facility off St. Mark's Square, have posted notices saying there will be no dry cleaning service until the summit ends because supply ships are not operating.

''This is insane,'' said Margaret Harrison, 62, a retired schoolteacher from London, as she struggled with two large suitcases, an umbrella and a handbag. ''I've been walking, or resting, for nearly an hour, and I am not even halfway to the hotel. Where are the bloody porters?''

Roberto Carrain, president of the association of Venice hotels, said Sunday was a bad day for tourists seeking ''sympathy and understanding'' from hotel operators.

''We have nearly 3,000 people arriving today for the summit, mostly government people and reporters who made their bookings months in advance,'' Carrain said.

As a result, many tourists visiting Venice have had to spend nights in hotels as far away as 60 miles from the city.

Richard Palmer, a vacationing business executive from Los Angeles, complained that he was awakened at 5 a.m. by ''shattering noises'' of helicopters landing on or taking off from a naval frigate anchored off St. Mark's Square.

''I thought the best security is invisible and quiet, but I guess I am wrong,'' Palmer said.

Security measures also have temporarily put out of business more than half of the city's nearly 400 gondoliers.

''It's a disaster,'' said Bruno Pamarini who represents 51 gondoliers off Riva degli Schiavoni, across from San Giorgio, and one of five gondola stations near St. Mark's Square that were ordered shut down for security reasons.

The gondoliers say they are demanding $175,000 from the government as compensation for four days of lost work.

During the last Venice summit in 1980, the city paid $120 dollars a day to gondoliers to stay away from restricted areas, but city officials say no such decision has been made this time.

There also are protests over high prices at hotels and restaurants. Local newspapers have carried extensive reports of complaints by foreign journalists of ''summit prices'' and rip-off joints.

''If somebody really paid 40,000 lire (33 dollars) for a bowl of spaghetti, he should have reported it,'' Carrain was quoted as saying in Il Gazzettino, a Venice daily. ''We would have taken the license away from that place.''

Prices in Venice always have been high, perhaps higher than any other European city, Carrain said. But he said a diligent tourist can find places where a complete meal can be had for as little as $12.

With commercial traffic cleared, the only boats plying the waters of a major section of the Grand Canal and adjoining waterways are gray or blue patrol vessels and motor launches chartered by summit delegates and journalists.

Airplane passengers were rerouted from Venice's Marco Polo Airport to the city of Treviso, 12 miles to the northwest, where they had to take buses or trains to enter the city. Venice's airport is open only to government and military traffic during the summit.

About 7,000 security troops and agents, including frogmen searching the waters around San Giorgio, have been mobilized for the summit.