With a slap at both President Carter and Congress and an appeal to Democrats assembled here for their 1980 convention, New York Mayor Edward Kock unveiled a new financial plan designed to eliminate a $268 million deficit in the current year's budget and a $733 million shortfall for the budget year beginning next July.

Koch proposed a 20 percent increase in real estate taxes and said he would trim another 1,200 jobs from the city payroll. The plan also envisions an increase in state aid this year and next as well as an additional $200 million in federal aid for fiscal 1982, which starts next July.

Koch was critical of both Carter, whom he supports for re-election, and Congress for failing to carry out pledges of aid both to New York and other industrial cities.

He said that federal mandates -- such as special education for the handicapped -- cost the city $1.15 billion this year, compared with $871 million in 1978, while unrestricted federal aid to the city has fallen from $447 million to $280 million.

Koch called on the Democrats to carry out 1976 and 1980 platform pledges to reform the nation's welfare system -- which would ease financial pressures on cities like New York.

At a City Hall briefing, Koch warned Carter that if he doesn't "carry the cities of this county . . . you aren't going to be the next president." He said that the city had been promised $512 million in federal funds it never received during the last two years.

New York also formally asked the Treasury Department for another $900 million in loan guarantees. The federal government, in an effort to keep the city from going bankrupt in 1978, agreed to guarantee up to $1.6 billion of the city's borrowing through 1981.

One of the requirements of the 1978 legislation was that New York must produce a budget that is balanced in standard accounting practice by fiscal 1981.

The 1,200 jobs Koch proposed to eliminate this year include 477 teachers and 181 policemen.

The so-called budget gap of $268 million in this year's $13.6 billion city budget occurred because the city spent more to settle with its municipal workers than Koch anticipated last January and because $100 million in federal aid he counted on did not materialize.

If the $200 million in additional federal assistance he wants for fisca 1982 is not forthcoming, he said the city would put in place another $50 million in budget cuts and boost city taxes by $150 million.