By day, Robert Rudnick is supervisor of assessments and taxation for Montgomery County, but last night he was just another Washingtonian who loves Halloween--and had begun celebrating it early.

"I've been dressing up like this for the past five years," said Rudnick, 41, as he showed his orange Afro wig and clown face.

Rudnick and Marna Zanoff, 37, were only two of the thousands of area residents who jammed downtown last night from Dupont Circle to Georgetown for the city's own version of Mardi Gras. Repeat performances are expected tonight and tomorrow night.

D.C. police barred traffic from Wisconsin Avenue, between K Street and Reservoir Road, and from M Street, between 29th Street and Key bridge. The same restrictions are to be in effect tonight.

Streets around the blocked-off area were congested throughout the night, with police reporting backups of a half hour to an hour. D.C. police also set up a roadblock last night at Wisconsin Avenue and Van Ness Street to check for possible drunk drivers. The checkpoint was expected to operate until dawn.

Rudnick and Zanoff were headed for one of the many masquerade parties held in the area last night and had stopped near Dupont Circle for dinner.

A legal secretary, Zanoff said this was the first time since childhood that she had dressed up for Halloween. Her rationale for joining the costumed throngs: "It's probably the only time of year when adults can act the way they really want to and not have to answer for their foolishness."

At Backstage, a performing arts store on P Street NW, owner Jean Rosenthal said, "It's been unreal all week," and estimated that her store had served 400 to 500 customers yesterday.

Though sections of 19th and 20th streets NW near Dupont Circle attracted their share of celebrants, most of the Halloween revelers seemed to be in Georgetown last night and with the streets closed to motor vehicles, hundreds of people spilled onto Wisconsin Avenue and M Street.

There were more gawkers there than costumed merrymakers, but the gawkers had plenty to look at. Bandits, harem girls, witches, E.T.'s, Laurel and Hardy, soldiers and Reagan look-alikes all vied for attention.

Police patrolled the area, but mostly they watched from the sidewalks. No major disturbances had been reported early this morning, and only two arrests--both for disorderly conduct--were reported by 1 a.m. Officials predicted that as many as 30,000 people would fill Georgetown streets by 3 a.m., when the bars close. About half that number is expected tonight, officials said.

Last night's crowd was peaceful, and most people appeared to relish the idea of being in their 20s, 30s and 40s--going on 8.

Nathan's bar-restaurant in the heart of Georgetown was decorated with orange and black balloons and had brought in makeup artists to paint the faces of customers.

Dan Callahan's long red robe and orb drew the most attention, especially since people thought he was costumed as the pope. Actually, he was dressed as the Infant of Prague.

"My robe is a bedspread, and I have a Christmas tree skirt, and I made the crown out of a bleach bottle," Callahan, 32, explained proudly. "The orb is a lamp shade painted gold and stuffed with a towel. And my cross is two paint stirrers thumbtacked together and painted gold."

Steve Stadelhofer, 25, of McLean turned heads as he walked into Nathan's. He had dressed up to look like the sailor in the Old Spice cologne commercial, but everybody thought he was a ringer for the late John Lennon.

"Nobody knows what I am until I showed the bottle of cologne," he said, slipping the product out of his coat pocket. "I guess they think I look like Lennon because of the dark glasses. Maybe I'll be John Lennon tomorrow night."

Robin Nickerson, 26, went as Cleopatra and spray-painted her brother Johnny Lee, 22, a silver metallic color so she could bring him along as a Martian. "I wanted to be a vamp and Cleopatra was very sexy, and she was in charge," Nickerson said.

Before the grownups took the spotlight last night, area children got in some merrymaking of their own. At Congressional Cemetery in Northeast Washington, children masquerading as witches, vampires, dragon slayers and clowns transformed grave sites into playgrounds for the cemetery's eighth annual "All Hallow's Festival."

Wearing plastic armor and carrying a toy sword, Howard Jenkins, 7, stood in the middle of the cemetery and roared and growled, "I am scaring people," he sneered. "Nobody can defeat me."