The police department has more narrowly defined the rules that allow protesters and other groups to parade in New York City, according to a notice of the changes published in the City Record last week.

The new rules require groups of 50 or more — whether on foot, bicycle, or other “devices moved by human power” — to apply for a permit before they can march on any street or roadway, the notice says. The old rule did not designate a numerical limit, an ambiguity judges used to throw out criminal cases against participants in the monthly Critical Mass bicycle rides.

Civil liberties advocates and activist groups said yesterday that they would fight the changes, which they said should be promulgated by the City Council. The new rules encroach “arbitrarily” on the right to assemble instead of clarifying the law, the head of the Assemble for Rights coalition, Mark Taylor, said.

The department gave notice in July that it was planning to define the rules more narrowly. An earlier proposal by the department required groups of 30 or more to apply for a permit, but after a public hearing in November, the department changed the number to 50.

The new rules take effect February 25. The next Critical Mass ride after that date is March 30, a day civil rights attorney Norman Siegel said will be a showdown over the new rules.