Two annual San Francisco festivals have instituted new clear-bag policies for their upcoming 2018 events.

Attendees of the Outside Lands Music and Arts Festival and the Clusterfest comedy event will be barred from bringing non-clear backpacks, purses, totes and drawstring bags larger than 4.5 inches by 5.5 inches onto festival grounds. The festivals — both produced by Superfly — quietly unveiled the new rules on their websites.

Festivalgoers may carry bags made of clear plastic, clear vinyl or clear PVC — not exceeding 20 inches by 15 inches by 9 inches. One-gallon clear Ziploc bags, small clutches, fanny packs and empty hydration packs are also permitted.

READ ALSO: Outside Lands adds Janelle Monae, Billie Eilish and more acts to 2018 lineup

"The safety of our fans is one of the few things we don't joke about," said a statement on the Clusterfest website.

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The Weeknd and Janet Jackson Headline Outside Lands 2018 Festival The line up for the music festival was announced Tuesday and it takes place in San Francisco's Golden Gate Park from August 10-12. Other notable artists performing include: Prior to Outside Lands, the Weeknd and Jackson will serve as headliners at New York's Panorama music festival. The festival is a celebration of music, comedy and local food an drinks from the Bay Area. Tickets go on sale Thursday at sfoutsidelands.com.

Media: Wibbitz

The Outside Lands announcement cited safety as a factor for instituting the new policy, as well as an expedited entry process to circumvent the long lines that have plagued the festival in past years.

Spokespeople for Outside Lands and Clusterfest did not immediately respond to SFGATE's requests for comment.

READ ALSO: Clusterfest returns to SF with Jon Stewart, Trevor Noah and Amy Schumer

A post about the new policy in a 10,000-plus-members Facebook group for San Francisco concertgoers drew dozens of angry comments on Tuesday night.

"Like they can't just keep searching our bags per usual?" asked Katie Sherman. "It's an all day event, with ridiculous weather. This is really dramatic, Ranger Dave!"

"A clear backpack doesn't provide you security," wrote David Motsonashvili in response to one commenter critiquing the outrage on the thread. "The downside is an eroding sense of privacy, a tacit approval of authoritarian security theater, and a reduced sense of trust in your fellow partygoer."

A handful of group members defended the move: "I don't think we should wait until something bad happens before we start asking for more safety and security, especially with the state of the world as it is," wrote Oriel Baesa.

Doug the Pug was a popular guest at the first Colossal Clusterfest in San Francisco. The Chronicle talked to his humans on Sunday, June 5, 2017.

Media: Mariecar Mendoza / sfchronicle.com

Outside Lands has taken over San Francisco's Golden Gate Park for one weekend in August for more than a decade. Clusterfest debuted in 2017 as the Colossal Clusterfest, a comedy and music festival. It's held at Bill Graham Civic Auditorium and outdoor stages in the San Francisco Civic Center Plaza.

Other Bay Area festivals continue to allow regular backpacks on event grounds, including Treasure Island Music Festival — produced by Noise Pop and Another Planet, and held this year at Oakland's Middle Harbor Shoreline Park — and Napa Valley's BottleRock, presented by Jam Cellars.

Bonnaroo Music and Arts Festival, Outside Lands' sister festival in Tennessee, also instituted a clear bag policy for 2018.

The Superfly festival rule change comes in a year that has seen 101 mass shooting incidents, according to the nonprofit Gun Violence Archive. The archive defines "mass shooting" as an incident involving the injury or death of four or more people in the same general location.

Most recently, a 17-year-old gunman opened fire at a high school in Santa Fe, Texas, killing 10 people. The Texas tragedy followed a February mass shooting at Marjory Stoneman Douglas High School in Parkland, Fla., where a teenage gunman murdered 17 people.

After the shooting, Stoneman Douglas required students to carry see-through backpacks and identification badges. Junior Kai Koerber told CNN he found the measure to be an invasion of privacy.

"It's difficult, we all now have to learn how to deal with not only the loss of our friends, but now our right to privacy," he said. "My school was a place where everyone felt comfortable, it was a home away from home, and now that home has been destroyed."

Michelle Robertson is an SFGATE staff writer. Email her at mrobertson@sfchronicle.com or find her on Twitter at @mrobertsonsf.