Brent Walls of the Potomac Riverkeeper Network says demonstrators plan a silent protest Thursday night at an open house that TransCanada officials are holding in Hancock to discuss the plan.
The company says the project would increase supply options and system reliability.
Walls says a gas leak would put at risk a drinking-water source for 6 million people, including residents of Washington, D.C., and its suburbs.
The Hancock Town Council agreed last month to allow the pipeline beneath town-owned land.
The National Park Service is considering a survey application for the Chesapeake and Ohio National Historical Park, which parallels the river.