Brenda J. Clayburn, a founder and later president of the City Union of Baltimore who was also a longtime city Police Department supervisor, died Sunday of undetermined causes at her Northwest Baltimore home. She was 63.

"She had recently been sick, and we are waiting the results of an autopsy," said her daughter, Shirley Y. Cooper, who lives in Baltimore.

"I was very saddened to learn of the passing of Brenda Clayburn. Brenda was a strong advocate for the thousands of city employees she represented, and she cared deeply for their welfare," Mayor Stephanie Rawlings-Blake said in a statement Monday.

"In all the years I had the chance to work with Brenda, she was a force to be reckoned with at the bargaining table, but she was also a kind-hearted public servant," said Ms. Rawlings-Blake. "I know that thousands of CUB members share a sincere debt of gratitude for her service."

"To sum it all up, labor really lost a champion on Sunday," said Ernie Grecco, president of the Metropolitan Baltimore Council of AFL-CIO Unions.

"She fought for her members. They were like family to her, and she always fought very hard for them," said Mr. Grecco. "She also had tremendous respect for other unions in the city."

The daughter of a career Army officer and a homemaker, Brenda Joyce Cooper was born in Newport News, Va., and later moved to Baltimore, where she graduated in 1967 from Frederick Douglass High School.

Ms. Clayburn attended Baltimore City Community College and Fortran Computer School.

She went to work in 1970 for the Baltimore Police Department as a clerk and rose to become a supervisor in the chief of patrol's office.

Ms. Clayburn, who had been a union activist for nearly four decades, was one of the founders in 1988 of the City Union of Baltimore Local 800 of the American Federation of Teachers, AFL-CIO, which is the primary bargaining unit for city and city public school employees.

She was the union's first elected executive vice president and was elected to the first of three terms as president in 2004, and subsequently in 2007 and 2010.

Ms. Clayburn was serving a third term as president when she died.

She had been on leave from the Police Department since 1998, when she joined the CUB as a labor relations specialist. She held that position until 2004, when she resigned to run for president.

Ms. Clayburn was a member of the AFL-CIO Community Service board and of the AFT Public Employees Program and Policy Council.

She was also a member of the executive board of the American Federation of Teachers-Maryland, and was a member of the executive board of the Metropolitan Council of AFL-CIO Unions. She was a trustee of the Baltimore City Employees Retirement System.

"I was truly saddened when I got the news Sunday night that Brenda had died. It was truly a shock," said Baltimore Comptroller Joan M. Pratt, who worked with Ms. Clayburn with the city employees retirement fund.

"She was the type of trustee who took her fiduciary responsibilities very seriously. She wanted to make sure that our money managers were getting the best return for the money that was invested," said Ms. Pratt.

"Brenda was definitely a voice for the voiceless. She always stood up for those who made less and gave the city much more than they were paid for," she said. "That is the type of person Brenda was."

Deborah F. Moore-Carter, the city's labor commissioner, is also a longtime friend. "She was a strong advocate for employees and made sure management did the right thing," said Ms. Moore-Carter.