Ferguson flash mob disrupts symphony

• The protesters launch into an old civil rights song


• They were all paying attendees, the symphony says


• The occasion "seemed almost destined," organizer says

(CNN) -- It was a protest of an altogether different sort.

Rather than take to the streets of Ferguson, these demonstrators took their demands to the seats of the symphony.

As the St. Louis Symphony returned from intermission Saturday night and readied to launch into Brahms' 'Ein deutsches Requiem' (A German Requiem), two audience members stood up and began singing an old protest song -- modified for a new cause.

"Justice for Mike Brown is justice for us all,

Which side are you on friend? Which side are you on?"

Then, others slowly joined in -- in the balcony, on the floor, in various parts of the auditorium.

The protesters unfurled banners. "Mike Brown 1996-2014," said one. "Racism lives here," said another.

The reaction was mixed. There was applause among many in the audience. Other patrons remained unimpressed.

"He was a thug," a man was captured on camera saying.

Brown, an unarmed 18-year-old African-American, was shot to death by a white police officer in August, fueling protests and spurring a debate on police use of force.

Investigation into Brown's shooting death complete

The genesis

The Saturday night protesters were all paying customers, said Adam Crane, the symphony's vice president for external affairs.

"There were about 50 people who had purchased tickets," he said.

The flash mob was the idea of Sarah Griesbach and Elizabeth Vega.

Two weeks ago, the pair were booed and handcuffed after hanging banners at a St. Louis Cardinals game.

"People were just outraged: How dare we interrupt a baseball game! They were saying really racist things," Vega said.

As they were escorted out of the stadium in handcuffs, Griesbach joked, "We probably need a different venue."

The idea for the symphony flash mob was born.

The performance

"We looked up the performances, and found (Saturday's performance) was a Requiem. It just seemed almost destined," Vega said.

The pair spread the word for strong singers through word of mouth and on social media. The protesters rehearsed the song three times before heading to the symphony.

"We wanted something simple that would stick with people," Vega said.

The singing lasted for about a minute and a half.

The outcome

As they left, the demonstrators chanted "Black lives matter."

Red hearts floated down from the balcony onto audience members below. "Requiem for Michael Brown, May 20, 1996 - August 8, 2014," said one side. Information on how to get involved was listed on the back.

The musicians on stage clapped.

"It was peaceful and it didn't interrupt the music," Crane said. "After the protest, they left Powell Hall marching two-by-two down the stairs and out the door."

This time, no one was handcuffed.

CNN's Joe Sutton contributed to this report.