Mark Silverstein, legal director for the American Civil Liberties Union in Colorado, questions why authorities are asking police and emergency workers to report "caches of objects" if they don't intend to act on the reports.

"They don't need 100,000 reports on who has bicycles and who has maps," he said.

"How do they decide a person getting a load of bricks is more worthy of investigation than another person with a load of bricks?" he added.

Denver police have been told to be on the lookout for stockpiles of a dozen items ranging from pipes to bikes -- things they say could be used to disrupt the Democratic National Convention next week.

A police bulletin sent to officers and other emergency workers asks them to look out for "medium and large numbers" of things that violent demonstrators could use as weapons.

PDF: Read the DPD Special Bulletin

Also on the list are shields, helmets, gas masks, chest protectors used in baseball or softball, protest sign handles, hand-held radios, chemicals and maps.

The memo gives some insight into what police are expecting during the convention.

Police said some of the items on the list have been used by protesters before during demonstrations at events like the DNC.

"The items have been found in various demonstrations to facilitate negative actions," said police spokesman Sonny Jackson. "We'd be remiss if we weren't aware or on the lookout for things."

The police are only being asked to be aware of stored items, and not necessarily to act, Jackson said.