Shawn Carney, a co-founder of the group, said it the practice began in Bryan, Texas, in 2004 because local activists wanted to demonstrate their opposition to abortion.
"We wanted to insert a new sense of urgency, that we can have an impact on the people who are having abortions in our community," he told The Tennessean.
Carney said at least 200,000 people have participated.
In Nashville, local organizers said at least 60 volunteers have agreed to pray outside the Women's Center, keeping the vigil going at least 12 hours a day through Oct. 1.
"We'll be praying and fasting for 40 days to close that place down," Vonda Spain said.
Observers say the movement has conceded Roe vs. Wade is unlikely to be overturned and an amendment to the U.S. Constitution banning abortion is also a dim prospect, so they are focusing on issues like government funding for abortion and state laws.
William Saletan, who covers abortion issues for the online magazine Slate, said the anti-abortion movement is "playing defense."
"And they are actually energized by playing defense," he said. "And it's all around the issue of funding for abortions."