The General Assembly is trying to pass a budget and finish its session within the next two weeks.
About 1,200 people attended the last rally, estimated General Assembly Police Lieutenant Martin Brock. Those arrested were charged with second-degree trespassing and violation of the building rules, Brock said.
The demonstrators first gathered outside and then marched two-by-two into the Legislative Building, armed with polling statistics and voter registration forms with which they conducted small "teach-in'' sessions designed to educate people about their movement and get more voters out to the polls.
Later they packed the hall on the second floor outside the House and Senate chambers, with the overflow crowd funneling upstairs into the third-floor rotunda.
The fifteen arrested were led off the second floor about 30 minutes after the rotunda rally began. Each held a red banner emblazoned in white letters with messages such as "Expand Medicaid.''
"We're small but we can still take you out at the polls!'' shouted Pattie Meegan, who was arrested Monday. She said her husband, who is fighting cancer, needs but doesn't qualify for Medicaid.
North Carolina is one of the states that has refused to expand Medicaid to broader income groups as allowed under the new federal health care law. Officials have said they don't trust the federal government to continue funding its share in the future.
The Moral Monday group chanted, shouted and heard speeches from some of those planning to get arrested. When General Assembly police asked the group to leave because it was disturbing the business of the House and Senate, the shouting only got louder.
"We're not going to be tricked now,'' said the Rev. William Barber, leader of the protests and president of the NC NAACP. "There's no rule about you standing in the people's house.''
The House briefly delayed debate on legislation as the voices grew louder outside the chamber's large metal doors.
"Moral Monday'' protests started when the Legislature returned in 2013. Organized by the NAACP and other civil rights groups, the protesters want to roll back a number of laws passed by the Republican-dominated government. They also want the state to expand Medicaid, pay teachers better and take other steps they said would improve the lives of poorer North Carolinians.
This year the protests have included sit-ins near Gov. Pat McCrory's office and in the office of House Speaker Thom Tillis.
More than 900 people were arrested at protests held during the Legislature's long session in 2013. About 100 have been arrested this year during the short session.
After Monday's arrests, hundreds of protesters marched downstairs and out the front doorway of the Legislative Building to Bicentennial Mall. The rally continued there with music and singing into the evening, eventually dwindling to about 50 people.
Barber congratulated the crowd on its 60th week of rallies, saying protesters have won in the courts on issues ranging from a restrictive voter ID law to Legislative Building rules aimed at limiting the demonstrations.
"Celebrate what you have done,'' Barber said. "You have built a movement, not just a moment.''