The Pittsburgh Urban Magnet Project needs a few hundred volunteers to create a massive public scene in Market Square Thursday.

To call attention to its "Stay & Play Fridays" promotion for Market Square merchants, PUMP wants as many people as possible -- "you and everyone you know," according to its publicist -- to show up at the Market Square stage at 6 p.m. today for a dry run of a mass freeze-in-place stunt. All volunteers will get Stay & Play T-shirts to wear when the signal sounds during the lunch hour on Thursday.

The "freeze" event -- a five-minute standstill of all participants -- will attempt to recreate a mission of the provocative international scene-making group, Improv Everywhere. Improv Everywhere has hundreds of participants it calls agents who show up for what it calls missions worldwide, promoting nothing but the fun of bewildering and fascinating other people.

Casey Mahaven, a spokesman for TRE Communications, the firm that PUMP hired to promote its Friday summer series, said she got the idea on You Tube, from a freeze event in New York's Grand Central Station.

The Pittsburgh event's mission is to help merchants in Market Square create a Friday evening scene of people staying Downtown after the work week ends. The series will begin Friday and run through Aug. 1.

The rewards will be food and drink specials and entertainment "that's more than just a rock band on stage," said Ms. Mahaven. "We will have bands, break dancers, tap dancers, a Chinese lion dance, a Polynesian fire show and a tailgating beanbag toss," with a different theme every week.

"The whole premise is that there are things to do Downtown. You don't have to leave the city at 5 p.m."

"We need about 100 volunteers to ensure that we get the most bang for our buck," said Erika Wirkkala of TRE Communications.

The signal for everyone to freeze is still being worked out, though it will probably be a car horn beeping in an agreed-upon code.

PUMP will distribute T-shirts to participants between 11 a.m. and 12:15 p.m. on Thursday at the fountain in PPG Plaza, then participants will go about their business in Market Square, wearing the T-shirts, until no later than 12:25 p.m. When the freeze signal sounds, everyone in T-shirts will be expected to stop in place, no matter what they are doing.

"People might be at the farmers market or at a cafe," said Ms. Mahaven.

In the Grand Central video, one man froze after dropping papers that scattered in front of him. One woman was lifting a spoon of yogurt to her mouth, and several people were looking at maps and guidebooks.

After the freeze, participants can stop back at the fountain for a gift card, compliments of PUMP and Dunkin' Donuts, the other sponsor.

Saying the more people who turn out the better, a swell of people might get $3 gift cards instead of $20 ones, she said, "but we hope people will want to do this anyway."

Volunteers can come out Thursday even if they miss this evening's run-through, she said.

"All you have to do is be there to get the T-shirt [Thursday] between 11 a.m. and 12:15 p.m.," she said.

To sign up or for more information, call or e-mail Ms. Mahaven at 412-253-0677 or casey@trecommunications.com. For the complete summer schedule, visit the Stay & Play Fridays Web site at www.pump.org.