Activist, organizer, and now author DeRay Mckesson sat down with Ayanna Pressley to discuss his new book “On The Other Side of Freedom: A Case for Hope” in front of a sold out crowd in Old South Church in Boston’s Back Bay Tuesday night.

Mckesson and Pressley were greeted with a standing ovation from the crowd of nearly 700 as they walked into the event hosted by the Harvard Book Store.

Mckesson wore his now iconic blue vest for the event. The former 6th grade math teacher began writing the book over a year ago and joked that it almost killed him. In the book, he writes about his work as an activist as part of the Black Lives Matter Movement.

Pressley, fresh off her September win over Rep. Michael Capuano and the prospective congresswoman-elect for the 7th congressional district, pressed McKesson on his political aspirations after his unsuccessful mayoral bid in Maryland.

“First I’m going to work in your office,” he responded.

Pressley asked Mckesson what a district attorney can do to combat mass incarceration, one of the issues that he is most vocal about.

"We can change the leaders but if we don't change the laws, the best thing we can do is yell at the laws," he said.

Among the diverse crowd were more than two dozen middle school students from the Brooke Charter School in Mattapan who wrote essays in class explaining why they wanted to meet Mckesson.

Syriana, a 6th grader at Brooke, said that after learning about civil rights leaders in class she was inspired to do something.

“I was like hearing something from an activist today not just Dr.Martin Luther King Jr., or like Rosa Park or someone like that — but someone who is actually in our community who is like standing up for black rights,” she said after the event, still wearing her school uniform.

A number of her classmates stood up and asked about gun violence, what inspired them, and how to be an activist.

“What would you say to a young activist ... like my age? What’s your message for them?” one young boy asked.

Mckesson’s answer is the last chapter in his book, which he says sums up the list of all the things he wished someone has told him in 2014.

“You’re enough to start a movement,” he said. “Start where you are and learn an issue well.”