Thousands of towel-waving, Philadelphia Flyers-hating, Sidney Crosby-adoring Penguins fans will flock to Mellon Arena tonight for a much anticipated playoff battle against the team's cross-state rivals.

And they won't even get in the building.

Such is the city's growing love affair with the young and talented Penguins that people are gathering in large numbers to watch the team's drive for the Stanley Cup on a 12-foot by 16-foot LED screen set up outside of the arena's Gate 3 entrance.

They are toting lawn chairs, blankets, picnic baskets and zeal, many decked out in jerseys of their favorite players, and crowding into the barricaded section off the main entrance at Washington Place and Centre Avenue, where the viewing is free.

Crowds have grown with the team's momentum, from several hundred fans in the Penguins' opening game against the Ottawa Senators to an estimated 3,500 for the team-clinching overtime victory Sunday against the New York Rangers, during which NBC, in its national broadcast, interspersed its coverage with shots of the cheering throng.

Team officials are expecting at least another 3,500 for tonight's opening game against the Flyers, and perhaps even more as the series progresses. The crowds have exceeded their expectations.

"It's become a phenomenon itself. People are coming down with beach chairs, picnic baskets. It's just great Pittsburgh stuff," Penguins President David Morehouse said.

"It's an extension of the 17,000 inside. It's basically indicative of what we've seen around Pittsburgh, which is basically this region, Western Pennsylvania, has embraced this team in a way we haven't seen before," he said.

"It's as close as coming and being inside the game as you can get without having a ticket and it doesn't cost you anything."

The Penguins estimate that they can fit up to 4,000 fans outside Gate 3. Already crowds have spilled over into the sidewalk on Centre Avenue and into the Marriott hotel plaza across the street from the arena. Mr. Morehouse said some fans are arriving well before the game to secure the best spots.

For tonight's game, the team will leave well enough alone. It has looked for ways to expand the viewing area, even walking around the arena one day this week to scout out other possible locations, but found "no space that works quite as well," Mr. Morehouse said.

"For right now, we're sticking to the plan. We'll see what happens Friday. If it grows to the point where we have to start thinking about other locations, there are only limited options," he said. "There's not a lot of space. We're victims of our own success."

Adding another screen outside of another gate could cause problems with crowd control and interfere with the efforts of ticket-buying fans to get in and out of the arena, he said.

One thing the Penguins don't want to do is move the screen off arena property.

"We thought about potentially putting the space somewhere else where there was more room. But we think that would take away from the game-day experience at the arena, where you can actually hear the crowd when goals are scored," Mr. Morehouse said.

As the crowds have grown, the Penguins have added more security outside. They also sell hamburgers, hot dogs and soft drinks, and will begin offering some merchandise with the start of the Flyers' series.

Alcohol is prohibited. The team also has asked fans not to bring grills or tents for safety reasons. So far, there have been no problems, other than the ever-tightening space, Mr. Morehouse said.

Ralph Cindrich, chief operating officer for DeBartolo Sports and Entertainment, which represents athletes and provides consulting services to businesses, said the outdoor venue creates "that much more excitement about what's going on with the Penguins."

"It sounds like one grand outdoor party," he said. "What a great atmosphere for people who don't have the opportunity or ability to purchase tickets to participate in that way with so many fans. I think it's a great idea."

The venue also could help to create an even greater audience for Penguins tickets once the new arena, with 18,500 seats, opens in 2010, Mr. Cindrich acknowledged. Mellon has 16,940 seats.

"It's a great marketing idea," he said.

Mr. Morehouse said the Penguins have gotten inquiries from other professional sports teams about the idea. In fact, the Dallas Stars have set up free outdoor viewing areas for their fans in their Western Conference Finals series against the Detroit Red Wings.

For the Penguins, the idea had its genesis with a fishing tournament, the CITGO Bassmaster Classic in Pittsburgh in 2005. A large screen was set up at the arena as part of that event. It caught the eye of Dave Soltesz and his sales and marketing staff, who brought the idea for an outdoor screen for the playoffs to Penguins management.

"We decided it was worth the expense," Mr. Morehouse said.

The concept is not new for a hockey team. The Senators, for instance, set up two giant screens outside of their arena, Scotiabank Place, for the conference and Stanley Cup finals last year. One Stanley Cup final game drew an estimated 7,000 fans, spokesman Brian Morris said.

Mr. Morehouse said the team is looking for ways to incorporate an outdoor viewing area in the planning for the new arena. He attributes the success of the venture to something more than Stanley Cup fever.

"What it shows is that Pittsburgh has become a full-fledged hockey town, as much as any town in the United States," he said.