No Democratic convention, not even a mini, would be complete without the usual assortment of party crashers, issue hawkers, publicity piggy-backers, and other hangers-on. Today, this city had a taste of all of them.

The most visible off-program event was played out a few yards in front of the main convention hotel, the Bellevue-Stratford, where several dozen black policemen and their supporters threw up a picket line to protest Mayor William Green's failure to increase the numbers of blacks on the squad.

The pickets booed Green lustily as he arrived for a cocktail reception, but the convention's host mayor chose not to rise to the bait.

"I feel as though we have invited people into our living room," said Green, who is anxious to use the event to showcase a gracefully aging city that is celebrating its 300th birthday this summer. "I don't think the family should quarrel while they have company in."

But the mayor's wife, Pat, showed no such reticence. Arriving a few minutes after her husband, the five-months pregnant Mrs. Green got into a shouting match with the pickets who were accusing her husband of being a racist.

Other interest groups chose different forums and strategies to make their points. "These things do attract people with causes," said Democratic National Committee spokesman Robert Neumann. "Some work the streets, others work the cocktail parties."

And still others work the media. More than 1,000 press credentials have been issued for the mini-convention, a number that seems to embarrass the working press in roughly equal the degree that it attracts those with a story to tell.

Republican National Committee chairman Richard Richards, for example, could not pass up the chance for a little free publicity in the enemy's lair, but found himself dismissed as "tacky" by his opposite number, Democratic National Committee Chairman Charles Manatt.

Meantime, the Americans for Democratic Action chose the occasion of the convention to hold a five hour issues briefing on its liberal agenda for the 1980s.

ADA national president Robert Drinan proclaimed liberalism "alive and well" and said, in keeping with the upbeat spirit of the convention, "I have never seen such unity in the Democratic Party in my adult life."

He added that he supported the thrust of the policy papers that the party has drafted for the convention, but that ADA members will offer a series of amendments to spell out more explicitly its support for jobs programs and its opposition to the tight money policies of the Federal Reserve system.

When the Association of Community Organizations for Reform Now (ACORN), a nationwide group, starts streaming into town today for its national convention, the tone will not be quite so congenial.

The group, which timed its meeting to coincide with the Democratic gathering, plans to stage rallies protesting new party rules which it says will exclude poor people from attending the '84 convention as delegates.

In addition to these groups, organized labor was out in force, so were the presidential candidates, putting together dry run convention operations with an eye toward getting the kinks out for 1984, and so were lobbyists, entrepreneurs, pollsters, campaign strategists, and hawkers of memorabilia.

So, to the chagrin of the DNC, was Lyndon H. LaRouche Jr., whose far right political organization, the National Democratic Policy Committee, issued a draft Democratic mini-convention policy pamphlet that, because of the name similarity, may have struck some delegates as being an official party organ. It is not.

"I'm afraid we attract Lyndon LaRouche like dogs attract fleas," lamented Neumann.