Stella Harlos stood in a corner of the crowded office and issued a stream of instructions in a firm, no-nonsense voice. "Put in your name, address and Social Security number," she said. "Your occupation goes here . . . What kind of income do you have? It goes on this line . . . "

After 30 minutes, Harlos was ready for the next group of taxpayers who jammed the downtown taxpayer assistance office operated by the Internal Revenue Service all day yesterday. Those seeking help with the standard 1040 Form were assigned to small groups of five or six--like the one led by Harlos. Other taxpayers trying to fill out the shorter and less complicated 1040A Form were placed in larger groups of 20 to 25.

Before it closed for the day, an estimated 800 people seeking last-minute tax help had moved through the office at 1201 E St. NW. Today is expected to be even more hectic at IRS offices in the Washington area as taxpayers and tax helpers rush to meet the midnight filing deadline for 1981 returns. To avoid late payment penalties, returns must be postmarked no later than April 15.

"It's zoo time," said Tom Keys, assistant chief of the IRS taxpayer service division for Maryland and the District of Columbia.

Keys said the group instructions offered this year are replacing the individual assistance that IRS offered in the past. "We do not prepare returns for the individual anymore, unless the person is handicapped, blind or illiterate . . . But they can come into the taxpayer office if they have a tax question, a problem or if they want to get self-help tax assistance. We put them in a classroom and go over the form line by line with the individuals filling out the forms themselves."

In addition to the downtown office, IRS offers taxpayer assistance at the Skyline Office Building, 5205 Leesburg Pike, Baileys Crossroads; Wheaton Plaza Shopping Center in Wheaton, and Penn Silver Office Building, 5408 Silver Hill Rd., Suitland.

So far, according to Keys, the taxpayer offices have helped fewer people than last year. One reason may be the switch from individual help to group help; another reason may be the downturn in the economy: "Many haven't had to file tax returns because of lack of work," he said.

But telephone calls for help are up significantly. Since Jan. 1 the Baltimore office, which receives phone calls from D.C. and Maryland residents, had answered 740,000 telephone calls--about 50,000 more than were answered during the same period last year, Keys said.

Many other calls never got through to the IRS. Keys said that his office has answered 12,000 to 15,000 calls each day this week. But another30,000 to 40,000 telephone callers counted each day by special telephone equipment received busy signals.

Because of the overload of calls, the IRS is asking that taxpayers seeking information about refunds wait until Friday or later to dial the IRS numbers. That would make it easier for taxpayers with tax questions to get through today and meet their filing deadline.

Residents of Washington as well as Montgomery and Prince George's counties can call IRS taxpayer information on 488-3100. The toll-free number for most of the rest of Maryland is 1-800-492-0460. Northern Virginians can dial 557-9230 for taxpayer help. The toll-free number for those in other parts of Virginia is 1-800-552-9500.

To handle the crush of last-minute taxpayer filings, IRS has assigned workers from other parts of the agency to answer telephones and man the taxpayer offices. Stella Harlos, for example, left her duties as a tax auditor to work this week in the downtown office and help people fill out their tax forms.

The U.S. Postal Service also has geared up for tax time.

In the main Post Office at North Capitol Street and Massachusetts Avenue, uniformed postal workers will be assigned to spots near the street where they will accept tax return envelopes from motorists until midnight tonight. The workers will be in uniform and will have signs identifying them as postal workers.

Postal workers also will be on duty until midnight outside the Merrifield Post Office, 8409 Lee Hwy., in Northern Virginia.

Suburban Maryland won't have postal workers on the streets to accept tax returns, but there will be midnight collections of mail deposited in outside mailboxes at four locations. Three of the boxes are in Silver Spring: 8412 Georgia Ave., 8051 Newell St., and 8616 Second St. The fourth box is in Prince George's County at 6511 Baltimore Ave. (old Rte. 1), Riverdale.