Instead of celebrating with flowers and candy, hundreds of people turned  out in San Francisco early Thursday evening to commemorate Valentine's Day with a pillow fight.

The annual "Great San Francisco Pillow Fight," now in its eighth year at Justin Herman Plaza at the foot of Market Street, traditionally begins at the stroke of 6 p.m.

The fight entails whacking others with pillows -- but organizers are reminding participants to only hit those who have pillows themselves.

A post on a Facebook page for the event suggests bringing a synthetic pillow, packing light and bringing water to stay hydrated.

The Department of Public Works is preparing for the feathers to fly, and for the mess that will create, department spokeswoman Rachel Gordon said.

"Feathers are not the easiest thing to clean up," she said.

Crews will be at the ready Monday evening, and are urging participants to be mindful not to leave a lot of debris behind.

Extra garbage cans will be placed in the plaza, Gordon said.

The department has plans to send a first wave of crews to the plaza with some steamer machines at 9 p.m., followed by a more thorough cleaning scheduled overnight.

At 6 a.m. Saturday, another crew will inspect the area and see if any more cleaning needs to be done, Gordon said.

Last year, cleanup costs were estimated at about $5,000.