BP Bridge, which connects Millennium Park to Daley Bicentennial Plaza, was eerily quiet a bit after the noon hour last Friday, despite the estimated two million fans who thronged nearby downtown streets, cheering the Blackhawks’ Stanley Cup win.

The winding, 925-foot, stainless-steel bridge is usually filled with office workers out for walks and tourists snapping photos during lunchtime in June. But Friday, the bridge was closed to pedestrian traffic, reportedly as a measure to control any crowds ambling over to the site from the Blackhawks rally at Michigan and Wacker.

The closure occurred on an afternoon when protestors, organized online, were to occupy the bridge for a brief “flash mob” against British Petroleum, the oil company whose Deep Horizon well is leaking tens of thousands of barrels of oil into the Gulf Coast.

“It seems pretty suspicious,” Sue Eleuterio, who came from Indiana to attend the protest, said of the closure. “It seems to me that word might have gotten out.”

Last week, messages circulated on the internet and via Facebook asking for British Petroleum protestors to mass at BP’s namesake bridge, bringing a black umbrella with them.

At precisely 12:11 p.m. that date, according to a note that circulated online, members of the flash mob were to “casually” converge on the bridge.

Nine minutes later, at the call of a whistle, the protestors would squat down, opening the umbrellas. “If you don’t have a black umbrella,” a message from the group read, “wear all black.” The silent protest — intended to make a statement that “swift action must be taken before more of our world is destroyed by oil” — would end by 12:35 p.m.

Flash mobs, created by a Harper’s Magazine editor in 2003, are sporadic, loosely organized public gatherings often made up of complete strangers and organized via the Internet or by word-of-mouth.

Mob participants often perform symbolic or sometimes even pointless acts and upon a final signal, disperse.

For J.B. Daniel, who has participated in several other flash mob events, the lure of this event was its creative statement.

“I liked the idea of being involved in a more visual protest rather than one that’s just lost among all the others,” he said.

Finding the bridge closed to the public last Friday, approximately 100 protestors with umbrellas improvised, altering their plan.

At 12:20 p.m. a whistle blew and the flash mob participants quietly and purposefully opened their umbrellas and sat or knelt on the concrete in front of the closed and guarded bridge.

Protestors remained mostly quiet. Participants carried out the protest without providing statements or propaganda supporting their stance, leaving curious passers-by questioning the group’s purpose.

“Do you know what’s going on here?” a bystander asked.

One group of onlookers caught on, yelling, “F— BP” and “BP sucks.” But despite questions and shouts, the protesters remained quiet, crouched underneath the black umbrellas.

When the second and final whistle blew at 12:35 p.m., the protesters immediately stood up, closed their umbrellas and silently left the bridge, meshing back into the crowds that had gathered in the area.