Everyone's finding out about 'best-kept secret' in East

''I went to Philadelphia but it was closed,'' W.C. Fields, a native Philadelphian, often quipped. Then there's the line ''First prize is a week in Philadelphia. Second prize is two weeks.''

Today, even the city's most avid promoters aren't above uttering a good-natured dig at the expense of their beloved city. But joke as they may, they are definitely having the last laugh. Philadelphia, the nation's fourth-largest city, is fast becoming a tourism ''supercity.''

According to the US Travel Data Center in Washington, the city's tourism and convention business here now provides 30,000 jobs, up dramatically from previous years. In 1980, tourism and conventions generated $1.16 billion for the economy.

''Now there's no question that it's a major industry here,'' says Sam Rogers of the Philadelphia Convention and Vistors Bureau.

''You can't compare Philadelphia to New York and you can't compare Philadelphia to Washington; it's a different kind of city,'' he says.

But up until a few years ago when the city began advertising its unique qualities, ''Philadelphia was the best-kept secret on the East Coast,'' another bureau spokesman says.

Yes, Philadelphians and tourists alike have loved Independence National Historic Park, ''the most historic square mile in America,'' with its Liberty Bell and Independence Hall. But what did they know of other landmarks, such as the Graff House at Seventh and Market Streets, where Thomas Jefferson lived when he wrote the Declaration of Independence? Or the Atwater Kent Museum, a museum devoted entirely to Philadelphia history, also on Seventh Street? Or the magnificent Powel House, built in 1765, one of the finest town houses in a city of fine town houses?

Here are some of the many ways the public and private sector are moving to turn Philadelphia into a tourism supercity:

* At a time when the budgets of many other agencies have been sliced, the Convention and Visitors Bureau's budget has remained at last year's level. The bureau is now bringing travel agents and convention planners from across the country to the city to drum up interest instead of sending its own staff out into the field, as it once did. Letting visiting travel agents see the city firsthand is a big plus.

In the next several months, the city will be unveiling its plans for a new convention center expected to draw millions more tourists to this ''best-kept secret.'' The convention hall project has the full backing of Mayor William Green, and it's now only a matter of time for the details, including cost studies, to be hammered out, top city officials explain.

* Bureau officials are working hand in hand with bureaus in neighboring suburban counties, such as the Chester County Tourist Promotion Bureau, in greater joint efforts to promote the attractions outside Philadelphia. People can visit the Liberty Bell and the Society Hill town house renovations downtown, and possibly later in the day be out in the ''country'' seeing perhaps the famous Longwood Gardens, the Brandywine River Museum, the Colonial Plantation at Ridley Creek State Park, or Valley Forge.

* Mayor Green and Commerce Director Richard Doran take every opportunity to boost tourism, in sharp contrast to the previous administration. National television news stories about police violence under former Mayor Frank Rizzo had kept many would-be visitors away.

Now Philadelphia has not only put this problem largely behind it, but for a city its size has a remarkably low crime rate. Another ''selling point'' Mr. Doran touts is the fact that Philadelphia rates 15th, behind New York, San Francisco, Washington, and other cities, on the cost of lodging and meals. According to Sales & Marketing Management magazine, the average daily cost of lodging and meals in New York in 1981 was $99.20, while in Philadelphia it was only $67.40.

* Four new luxury hotels are going up in the downtown area. And existing hotels are continually being upgraded and renovated.

Just a few short years ago, a constant complaint of out-of-town businessmen was that there were only a few ''quality'' places to stay. Thus, many would try to wrap up their deals in a day and take a night plane or train back home. Besides the marked improvements in accommodations, Philadelphia's restaurant picture is fast emerging as one of the brightest in America.

Philadelphia is second only to New York now in legitimate theaters, ahead of Boston, Chicago, and Los Angeles.

Perhaps one of the city's best selling points is its ''small town'' feeling. You have the big-city attractions from pint-size landmarks to gleaming 20 th-century office towers. But you can walk, drive, or ride a bike to them with the greatest of ease, and continue on, by bike or car or bus, to the suburban and country sites. Oh, you might run into an occasional traffic jam or two, but this might serve to remind you of the bigger, louder ones you left behind at home.