Hammer and sickle flags are popping up frequently in ports around the world these days, spreading anxiety and outrage among Western shippers.

By slashing cargo rates up to 40 per cent on some sea routes, the Soviet Union's bustling merchant fleet has grabbed so much maritime trade that European shipowners are having a hard time staying afloat.

Western shipping companies complain that Soviet vessels are trying to "dump" competition by undercutting world freight prices. They also warn that the Soviet methods are motivated, in part, by the Kremlin's geopolitical designs.

The state-run Sovinflot shipping agency beats its capitalist rivals in a number of ways. Some of its sailors are paid only $31 a month and subsidized fuel costs only a quarter of world oil prices.

Moreover, the vast Soviet land mass that straddles two oceans and two continents provides critical advantages in handling international trade.

For the lucrative traffic between Asia and Europe, the Trans-Siberian railroad is quicker and 20 per cent cheaper than the shortest route - by way of the Indian Ocean and Suez Canal - plied by Western freighters.

Compared with the Cape of Good Hope passage used by huge oil tankers, the Soviet railroad cuts time, distance and shipping charges by more than one-half.

Over the last decade, the Soviet merchant fleet has quadrupled to about 8,000 ships. Its price-cutting led to a 74 per cent surge in cargo carried between 1970 and 1975, and another 30 per cent gain is expected by 1980, according to a report drawn up by the Common Market's executive commission.

The Soviets also insist on carrying most trade in and out of their country. Last year their boats transported 70 per cent of all trade between the Soviet Union and the European Economic Community.

Britain, West Germany, The Netherlands and Belgium have filed private protests with Moscow and are seeking to lure the Soviets into the various shipping agreements that set cargo prices on global trade routes.

"It's in the Soviet Union's own interest to join the conferences and play by the rules," says Pierre Pluys, head of the 29-ship Belgian Maritime Co. "They stand to gain more money that way rather than undercutting market prices to everybody's loss."

In the past, Sovinflot made some conciliatory moves toward the international agreements, but it quickly resumed its maverick ways when chances for fast profits appeared on several routes.

Lately, Soviet ships have been avoiding heavy, cheap cargo and concentrating on such profitable freight as appliances, cameras and electronic goods - depressing even more the losses of Western shipping companies.

The growing Soviet maritime strength has not escaped the attention of NATO security experts, who note the possibility that Soviet control over key shipping routes could imperil vital supplies in a sudden crisis.

They point out that the enhanced profile of the Soviet merchant fleet coincides with a dramatic build-up in its navy.

European shipowners, however, have a more immediate concern - that the need for hard currency will tempt the Soviets to resort to even more cut-throat shipping tactics.

Karl Heinz Sager, director of West Germany's Hapag-Lloyd Lines and president of the Common Market's shipowners association, labels the Soviet pricing practices "one of the most dangerous threats facing Western shipping companies."

Sager and other European shippers have demanded that the Common Market impose quotas on Communist bloc ships that engage in pricing practices they consider unfair. EEC officials are wary about provoking Soviet retaliation in other economic sectors and harming the precarious state of detente.

Western sea traders want to reach some kind of understanding with the Soviets before West Germany finishes the Rhine-Main-Danube canal in the early 1980's. That will enable Soviet ships to sail up the Rhine and undercut business on inland routes as they have done with oceanic shipping.