Little monsters decked out in costumes and seeking treats will swarm with their parents through downtown streets in Port Townsend, Sequim and Port Angeles on Monday.

In Port Townsend, Halloween will begin with a parade.

Preschoolers through sixth-graders and their ­parents can assemble at 3:45 p.m. at the 1st Security Bank Plaza at 734 Water St. for the Port Townsend Main Street’s 22nd annual Main Street Downtown Trick-or-Treat and Costume Parade.

Friendly Main Street Witches will lead off the parade at 4 p.m., headed for Quimper Mercantile, 1121 Water St.

Children can trick-or-treat on the way back until about 5:30 p.m.

Water Street and some side streets from Madison to Polk will be closed to traffic from 3:30 p.m. to 5:30 p.m.

Some 1,500 to 2,000 people are expected to attend.

In Sequim and Port Angeles, children are welcome to receive treats at participating merchants’ stores on Halloween afternoon.

The annual Sequim Merchants Trick-or-Treat will be between 3 p.m. and 5 p.m.

The event, sponsored by the Sequim-Dungeness Valley Chamber of Commerce and Sequim merchants, features business owners with a posted picture of a pumpkin in their window handing out treats to children.

The Port Angeles Business Association will host Downtown Trick-or-Treat from 2 p.m. to 5 p.m.

Children and parents are invited to visit downtown Port Angeles for dozens of participating businesses with goodies, giveaways, photo shoots and other activities.

Other Halloween activities have been planned all weekend, including some today and Monday.

For a full list of Halloween events on the North Olympic Peninsula, see http://tinyurl.com/PDN-peninsula halloween.