CHICAGO—After violent protests erupted in cities across the globe last year, U.S. cities hosting some of 2012's most contentious events are changing their laws to try to head off trouble before it starts.

In Chicago, host of the overlapping North Atlantic Treaty Organization and the Group of Eight summits in May, Mayor Rahm Emanuel has proposed limiting the hours of demonstrations, tightening permit rules and jacking up fines. The minimum penalty for violating parade rules, for example, would soar to $1,000 from $50.

In...