A fiery crash on the Cabin John Bridge involving a tractor-trailer and a car seriously burned two people and halted an estimated 20,000 commuters on the Capital Beltway at the start of yesterday evening's rush hour.

Rescue workers said they believe a third person--the father of the truck driver--may have fallen from the truck's cab into the Potomac River or jumped to avoid the flames. A search by police divers was called off temporarily when a sudden storm swelled the river's current early in the evening.

The accident, about 3:30 p.m., halted traffic on the bridge, which connects the beltway's legs in Montgomery and Fairfax counties. Thousands of commuters spent hours finding alternative means of getting to their destinations over roads jammed with unaccustomed traffic.

Maryland State Police reported early today that an inspection disclosed no structural damage to the bridge and said they hoped to have all lanes back in use by the morning rush hour.

Although police diverted traffic onto alternative routes, including the George Washington Memorial Parkway on both sides of the river, backups reached epic proportions. In Maryland, the beltway jam was reported to stretch 15 miles at one point.

Virginia officials lifted car-pooling requirements on Interstate Rte. 66 inside the Beltway in an effort to ease the problem.

The accident sent thousands of commuters to different river crossings, and the usual rush-hour crush was compounded on bridges downstream. At 7:30 p.m., traffic was reported at a standstill on both Chain and Key bridges, with five cars disabled by overheated engines on Key Bridge.

Some motorists crossed to the beltway's opposite lanes and circled 30 miles to the Woodrow Wilson Bridge, which connects the Maryland and Virginia Beltway to the south, according to Maryland highway officials. The effects of the bridge blockage were felt on a number of key commuter routes, including Wisconsin and Connecticut avenues in Montgomery County, which were reported unusually jammed with traffic headed into Washington.

About 8 p.m., one lane of traffic was opened in each direction on the Cabin John Bridge's inner span, but at midnight police reported there was still a two-to-three-mile backup in each direction.

The day's highest official temperature was 95 degrees, and in Montgomery County officials issued a bulletin urging motorists trapped in the traffic jam to turn off their car motors and find a shady place to sit in order to avoid both mechanical and health problems.

The accident was still under investigation last night. Police said the truck, carrying a load of plastic covers for fluorescent lights packed in wooden crates jackknifed and caught fire on the bridge's Virginia-bound lanes. Its cab went through the bridge's guard rail and hung over the river more than 60 feet below.

The truck driver, Peter Jay O'Donnell, 30, of Mason City, Iowa, was flown by helicopter to the Medstar unit of the Washington Hospital Center where he was reported in critical condition with second- and third-degree burns over 95 percent of his body.

His father, John P. O'Donnell, 57, was missing last night and police said they believed he either jumped or fell from the blazing truck into the river, according to Officer Thaddeus Ripa of the Maryland State Police.

The car involved in the accident, a 1979 Toyota, was driven by Robert A. Rich, 24, of 957 Clopper Rd., Gaithersburg. He, too, was flown to the Hospital Center by helicopter and was reported in fair condition with second- and third-degree burns. Police said Rich, a manager of Chi-Chi's Mexican Restaurant in Springfield, was the sole occupant of the car.

Firefighters ran out of foam to quell the truck blaze at one point and at 6 p.m. decided to let the fire burn itself out.

The traffic jam prevented tow trucks from reaching the scene for nearly two hours.

The Cabin John Bridge, formally named the American Legion Memorial Bridge, normally carries 120,000 vehicles a day, according to the Maryland Highway Administration. On July 13, a 3-by-5-foot section of the bridge's deck fell out, and a 12-mile backup resulted during the morning rush hour.

The bridge is scheduled for redecking in 1985.