DENVER -- Denver police have been asked to be on the lookout for stockpiles of gas masks, shields, cases of nails and bikes that could be used by violent protesters during the upcoming Democratic National Convention.

"History has shown that violent demonstrators collect and stockpile items of supplies at various locations to be used to further their cause at these types of events," said a bulletin released to officers last week.

Officials predict anywhere from 20,000 to 50,000 protesters will descend on the Mile High City during the DNC, being held from Aug. 25 to 28.

Officers have been told to be on the lookout.

"All first responders need to be alert for medium and large numbers of these items that seem out of place for its location," the bulletin said. "This could indicate the stockpiling of supplies to be used against first responders." i

PDF: Read the DPD Special Bulletin

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

A spokesman for the ACLU said it will have a news conference on Thursday to discuss the bulletin. ACLU Executive Director Cathryn Hazouri and legal director Mark Silverstein will speak.

Also scheduled to speak is a Denver activist who the ACLU said was subjected to police questioning about bricks being unloaded at her house.

"Although the bricks were acquired for masonry repairs, Denver police accused the activist of 'stockpiling' the bricks for the DNC," the ACLU said in a news release.

The news conference will be held at the ACLU office at 3 p.m.

This bulletin sent to police includes potentially dangerous items like signs with forward-facing screws that "while not deadly ... have caused puncture and slashing wounds on officers."

Also on the list are helmets, maps and bicycles, which the bulletin said "are used to blockade sidewalks, streets and can be used to slow down responding emergency vehicles."