A popular Cape Cod beach has been closed for swimming after a great white shark estimated at 20 feet long was spotted leaving Chatham Harbor.

The shark was spotted and tagged on Thursday.

Niko Chaprales, captain of a shark hunting boat, tells the Cape Cod Times he spotted six great whites off Chatham on Thursday, including the 20-footer he described as the biggest he's seen.

In response, officials closed Lighthouse Beach in Chatham, as well as all east-facing ocean beaches along the Orleans/Chatham line, from Nauset Beach to Monomoy.

There have been numerous shark sightings in Cape Cod waters this summer, and one man was bitten by a shark while swimming off a Truro beach.

Experts say sharks are attracted to the area by an abundance of seals. (AP)