Holiday Treats--and Trick : Disneyland: A monster traffic jam during morning rush keeps many from joining free fun that park promised anyone in Halloween costume.

Disneyland hosted a Halloween megabash for nearly 50,000 people Monday, but the real horror show was outside the gates.

The meanest, most bloodcurdling creatures this side of Transylvania may have been the folks who donned costumes to qualify for the promised free admission to the amusement park only to be turned away when the titanic traffic jam they helped create prevented them from getting to Disneyland before the parking lot closed at 8:30 a.m.

“We were stopped bumper to bumper for an hour and 45 minutes. It was disgusting,” said Helen Caswell of Chino Hills, who arrived too late to deliver three grandchildren, ages 5 to 11 and dressed as a ballerina, a cowboy and an Indian. A 15-year-old grandson and his friend had bolted from the car and bought admission tickets.

“The little ones were disappointed,” Caswell said. “Real disappointed.”

Rush-hour traffic throughout Orange County was slowed for more than an hour, and the southbound lanes of the Santa Ana Freeway got so badly knotted that authorities dispatched trucks with signs warning of delays ahead.

“It became a virtual parking lot from about 6 a.m. to 9,” said CHP Officer Ron Johnson, whose electronic traffic-management monitor was lighted in red--indicating that the flow of freeway traffic approaching the park was stuck below 20 m.p.h. “It was all red. It was unbelievable.”

Event co-sponsors estimated that 50,000 party-goers arrived in time for the park’s first-ever Halloween fest, and the liveliest were already waiting when the lots opened at 5 a.m. Park officials didn’t know how many were left out.

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But a spokesman said Disneyland stuck to its promise to let in free anyone in costume who was at the main entrance by 8:30.

“We’re sorry for anyone who didn’t enjoy Halloween with us,” spokesman John McClintock said. “The rules were pretty clear.”

The event was co-hosted by Los Angeles radio station KIIS-FM and promoted for a week on host Rick Dees’ morning show. The publicity paid off with crowds that swamped local streets and freeways, already bulging with rush-hour traffic.

“When you have a major event coupled with rush-hour traffic, you’re going to have delays,” said Anaheim Police Sgt. Fred Roush, who had the unenviable job Monday of supervising local traffic control. “There’s a saturation point.”

Inside the park, ghouls, witches and machete-toting murderers gathered at the Haunted Mansion, joining the closest Disneyland has to a scary crowd--Captain Hook, a couple of evil queens and Jafar of “Aladdin.”

“We’ve never really done anything. We decided if we were going to do something, it should be pretty big,” McClintock said.

Caswell packed her brood back into the car for consolation stopovers at the Disneyland Hotel--not much going on there, it turned out--and a nearby restaurant for lunch and ice cream.

“We’ve never tried anything like this before,” she said later. “I wouldn’t try that again.”