Motorists who park their cars in Cambridge, beware. The Denver Boot is coming to town to clean up on overdue parking fines.

Beginning March 24, the city will start slapping the clamp-like boot on the cars of serious scofflaws--those with 15 or more outstanding tickets, George Teso, Cambridge director of traffic and parking said yesterday.

There are about 432,000 unpaid parking tickets in the city this year alone, and another 700,000 from previous years backed up in the courts, Teso said.

When attached to the tire, the boot renders a car immobile by clamping onto the rim of the tire and locking it in place. The boot also covers the hub cap, thus preventing a motorist from merely taking off the locked tire.

In addition to paying all their outstanding fines, drivers who get booted must pay a $25 removal charge, plus an extra $5 for each day the boot is on the car.

The city may also give the boot to any driver who amasses five delinquent tickets in a year, but probably will restrict itself at first to offenders with many more, Teso said.

Trying to drive the car with the boot on it will not only damage the car extensively, but, if caught, makes the driver liable for a $500 fines, Teso said. A mechanically inclined person might be able to remove the boot, within a few hours, but would probably get caught in the act, Teso said.

The boot is part of a streamlined effort to collect delinquent parking fines, made possible by a statute passed as part of the 1982 state budget, which allows city officials to impound cars directly instead of waiting for a court order, Teso said.

The city stands to gain about $1 million from the new procedures, boosting annual traffic revenues to about $3 million, Teso said.

The money currently collected from impoundment fines goes to pay for towing service, Teso said, but with the boot, towing isn't necessary, so the city keeps the extra money.

Under another new statute aimed at minor violators, a $5 ticket that goes unpaid for 21 days automatically becomes a $10 ticket. If the payment is still neglected after a warning letter is issued, the fine jumps to $25 and the city notifies the registry of motor vehicles.