MCI Communications Corp. plans to introduce residential long-distance telephone service to the Washington area within 90 days, company officials said yesterday.

This will mark the first time a Bell System competitor has challenged the telephone giant in the substantial long-distance residential market here.

Impressed with sales figures in Denver and Cincinnati where the service was introduced this month, MIC plans to offer the service here by the end of June and in at least a dozen other markets by the end of the year.

MCI -- which is based here -- has sold its long-distance service to more than 2,000 customers in the Denver area and several hundred more in Cincinnati. A massive MCI advertising blitz began in Denver on March 3 and in Cincinnati a week later.

The advertising campaign, which MCI expects to spread to the Washington area, stresses the economic advantages of the new system: the company says it cuts the cost of an hour-long, 500-mile call by about 45 percent.

The ads printed in Denver directly challenge the Bell System's own advertising campaign. "Reach out and touch someone," the MCI ads say. But do it for half of what Bell charges."

The MCI television ads to Denver take a similar tack, showing people talking on the telephone as the cost of the call flashes on the screen compared with Bell's costs. The example in the television ads says a 300-mile, 27-minute interstate call costs an MIC subscriber $3.07 compared with $6.05 for a Bell-System customer.

Using a push-button phone only, an MCI customer first connects with the seven-digit number of the MCI computer. Then, the customer pushes his or her five-digit personal code. Finally, the caller dials the conventional 9- or 10-digit long-distance number, and the call is put through.

MCI charges these residential customers $10 a month to hook up with its computers and suggests that unless a consumer has a long-distance bill of more than $25 a month, its long-distance service might not be worth-while.

"If you're kind of person for whom long-distance calls have become a necessity, nothing's come along that can be as much help to you as MCI," a company ad says.

MCI Chairman William McGowan said the success of the Denver program -- which features as many as eight or nine television ads a day and a print ad of each Thursday -- prompted the company to step up its timetable for extending the marketing of the service.

McGowan said that the growth of the service depends on MCI's ability to expand its microwave telephone networks by constructing new systems, hiring the needed personnel and securing the needed computers.

"I'm surprised that the spread of the service and the initial response on the part of people in Denver has been so good," McGowan said.

McGowan also pointed out that Denver is a particularly good city for the long-distance telephone business because so many of its residents are recent migrants to the growing Colorado metropolitan area.

But McGowan also is encouraged about MCI's prospects for the introduction of the service in Washington, thought also to be an important long-distance market because of many of the same factors that seem to contribute to MCI's success in Denver.

In addition, Bell System and MCI studies also indicate that long-distance telephone use is particularly popular among young people, products of the so-called Baby Boom years. They write fewer letters than their parents, for instance, communications experts say.

MCI currently serves more than 40,000 companies and had revenues last year of about $95 million.