Williams said she was handcuffed after she “refused to disperse” from the rotunda.
Williams is charged with obstruction, Capitol police said. She was released on a signature bond after being detained for about five hours.
Williams said she felt as though she was targeted for standing with protesters concerned about voter suppression.
“I stood with constituents to demand that their voices be heard and countless other Georgians who cast ballots on last Tuesday and thought that their votes were counted and are learning now that they’re not,” she said after her release. “I will continue to stand with the citizens of Georgia — and any citizen — to demand that their votes be counted, because that is the bedrock of our democracy.”
The other 14 protesters are charged with disrupting the General Assembly.
The protest in the rotunda under the Gold Dome was organized by a local Black Lives Matter group to pressure state officials to ensure all absentee and provisional ballots are tallied in the governor's race between Stacey Abrams and Brian Kemp. Kemp has declared victory.
Occasionally, the group of roughly 100 people broke into chants of “count every vote.”
Authorities said the demonstration was broken up after several warnings because of rules that prohibit chanting or yelling while lawmakers are in session. Police armed with stacks of plastic restraints circled the protesters as the clocked neared 1:30 p.m.,when the House convened for a special session called by Gov. Nathan Deal.
Capitol police said they arrested 15 people, including Williams, who are accused of violating state code that prohibits disruption of “orderly conduct of official business.”
Williams and other protesters were taken to the Fulton County jail, where lawmakers and supporters gathered demanding that all 15 people be released without charges.
State Rep. Park Cannon, D-Atlanta, called the arrests a "travesty."
“It’s funny because one in 18 Georgians is under correctional control in the state of Georgia,” she said. “We see this as indicative of the wrong trends in Georgia as it relates to fair elections and people being able to feel safe in their communities.”
Williams’ Senate colleagues condemned the lawmaker’s detention.
"When a sitting senator, who is the vice chair of the state Democratic Party, is thrown into a paddy wagon at the state capitol it is a stark reminder that our right to freely assemble is at risk," said state Sen. Nan Orrock, D-Atlanta.
Williams is the first vice chair of the Democratic Party of Georgia. Party Chairman DuBose Porter said Williams was arrested “for doing her job where she works.”
"Today, (Williams) was arrested at the Georgia State Capitol while standing up for her constituents' right to peaceful protest and advocating to count every Georgian's vote,” Porter said. “We stand with her and with all Georgians whose Constitutional rights are at risk. “
Georgia includes a provision requiring that legislators “shall be free from arrest during sessions of the General Assembly” except for treason, felony or breach of the peace.
Abrams campaign manager Lauren Groh-Wargo criticized legislative leaders for arresting protesters concerned about voter suppression.
“Today there were people who came to the Capitol to raise this issue,” Groh-Wargo said. “They were literally only asking to be heard. Demanding that this state count every vote.”
She thanked Williams for standing with the protesters.
“I applaud her bravery and we stand with voters and their story and demand that they count every vote,” Groh-Wargo said.
Staff reporter Tia Mitchell contributed to this report.