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The idea of opposite may not apply. In Keynes's general theory, classical microeconomics is a special case which applies when its assumptions are met. In modern times, Milton Friedman was probably the most prominent of the non-Keynesians. The University of Chicago Economics Department is identified commonly as classical and anti Keynesian.

The monetarists like Dr. Freidman and also supporters of Austrian school. of course., Marxists too.

Keynes has not deviated from classical and neo-classical theories and upholds state so they aren't.

Few realize that monetarism is anti-state and near to Austrian school. Marxists do not recognize existence of market which is not state owned.

Classical and monetary and institutional economists.Do you want names? For a more simplifier version, it is called a Freshwater against the Saltwater in the US.