Russia has ordered the United States to end its financial support for a wide range of pro-democracy, public health and other civil society programs here, in an aggressive step by the Kremlin to halt what it views as American meddling in its internal affairs. The Kremlin’s provocative decision to end two decades of work in post-Soviet Russia by the United States Agency for International Development — with little warning ahead of an Oct. 1 deadline — was announced on Tuesday by the State Department in Washington. The move stands to cut off aid that currently totals about $50 million a year, a relatively small sum but a potentially devastating blow for groups that came to rely on foreign money as domestic controls over politics tightened.