It's inhospitable, possibly uncivilized and maybe even downright un-American. San Francisco, with the peak tourist season just getting under way, has temporarily shut down its fabled cable car system because of a mechanical breakdown and is proposing an extended shutdown next year.

The long Powell Street corridor that stretches from Market Street to the top of Nob Hill appeared almost ghost-like in its emptiness recently as tourists stood dismayed and disbelieving at the system's deserted Powell and Market turntable. The system has not operated this month, but service is now scheduled to be restored July 4.

And neither was there any joy at the Convention and Visitors Bureau.

"We've had calls from travel agents and convention groups expressing dismay and even suggesting they might postpone their visits, as if the city had nothing else to offer," bureau official Marge Booker told the Los Angeles Times.

The latest problem with the rickety but popular system occurred when an 18-foot-long steel shaft more than a foot thick, part of the main driving mechanism for the city's three cable car lines, cracked, causing the cars to stop in their tracks.

Richard Sklar, acting general manager of the city Public Utilities Commission, attributed the breakdown to "fatigue failure.... It's like bending a clothes hanger. You can bent it once but if you do it back and forth 25 times you have a problem."

He said that a new shaft has been ordered from a Los Angeles steel company and that crews will work around the clock to get the system operating as soon as possible.

Earlier, it had been announced that the city would be minus its trademark cable cars between October 1980 and May 1981, while the machinery that propels them is shut down for a $30-million renovation. There also will be reduced service for 12 months starting this October while the cable car barn is rebuilt.

Under the renovation plan, the cable cars will retain their romantic, old-fashioned look, but their wheels, brakes and cable-grip mechanisms will be redesigned and modernized. The plan is to make the 106-year-old system, designed in 1873 by Andrew Hallidie to navigate the city's steep downtown hills, safer and more reliable.

In recent years, the cable cars have been frequent targets of critics here, who contend that they are unsafe, inefficient, costly and should be replaced with a more modern transit system. But such attacks are always met with torrents of opposition and beaten down.

Rod Bartholomew, a city transit official, said of the city's four public transportation systems - cable cars, streetcars, buses and electric trollies - that the cable cars are the "most cost efficient" in terms of the city subsidy it takes to operate them. They return almost 60 cents in fares for every $1 spent on them, he said.

But the city and its insurers also have paid out millions of dollars in damage claims to people who have been injured when the cars have occasionally careened out of control, jolted to a quick halt or flung passengers off on sharp curves. One woman won a $50,000 judgment a few years ago when she asserted that her injuries in a cable car accident caused her to become a nymphomaniac.

Bartholomew said, however, that contrary to popular opinion, the cable cars get heavy usage not just from tourists but also from San Francisco residents. "The 26 cars haul 14 million people a year," he said. "We've got 105 streetcars and they haul 15 million."