The first African-American woman to serve as a police chief in Virginia wrote in a four-page letter Monday that she was forced to resign in a move she believes was racially motivated.

"I can assure you that I did not 'quit' on the citizens of Portsmouth. My mother did not raise me to be a quitter," wrote the former Portsmouth police chief, Tonya Chapman. "She raised me to be a strong woman. As such, my resignation was not tendered under my own volition. This was a forced resignation and our City Manager was the conduit."

Chapman, who became chief of the city's police force in 2016, said she met some resistance in her efforts to change a police department culture that, she said, included "bias and acts of systemic racism, discriminatory practices and abuse of authority."

"Some quite frankly did not like taking direction from an African American female," Chapman wrote.

These "racial tensions" within the department became "blatantly apparent" after a white officer fatally shot a black man in 2015, Chapman wrote. The officer in the incident was fired and indicted on a charge of first-degree murder, but was later found guilty of a lesser manslaughter charge.

Portsmouth is a city of about 100,000 people, of whom more than 50 percent are black.

Chapman also wrote that for two years, "members of a highly influential fraternal organization" have tried to generate a vote of “no confidence” in her, "without success, as they have not been able to articulate valid reasons."

She said a former black male chief of the department had been similarly targeted while, in contrast, a white chief who "turned this department upside down" was not.

The Portsmouth Fraternal Order of Police said in a statement that "at no time did the Portsmouth FOP call for or attempt to call for a vote of no confidence" in Chapman.

The police union continued that it "was willing and wanted to establish a good working relationship with Chief Chapman for the benefit of the department" and will continue on that path with the new, interim chief.

A statement from the city said Portsmouth City Manager L. Pettis Patton accepted Chapman's resignation on March 18, and Assistant Chief Angela Greene was named interim police chief.

NBC News reached out the city and Patton for comment. Dana Woodson, public affairs officer for the city, responded, "We do not comment on personnel matters."

Chapman said when she met with Patton last week, "She began to read a scripted document that stated in part that I had lost the confidence of my department." When Chapman asked for further details from Patton, whom she said she had an "extremely close professional relationship" with, Patton "continued to state that she could not say anything more."

"She then stated that if I did not sign the pre-written letter of resignation, she would terminate me," Chapman wrote. She said she was promised two months severance pay if she resigned, so she signed the letter "under duress."

"Citizens of Portsmouth, I ask you, if I had done anything to warrant my immediate dismissal, would I have been offered a severance?" Chapman wrote in her letter.

Chapman said that in the first year of her three years as chief, homicides decreased in the city by more than 50 percent. She also said that under her leadership, minority representation at the Portsmouth Police Department rose to 31 percent. The department employs more than 350 personnel, including 255 sworn officers.

She said she was requesting a positive letter of recommendation from the City of Portsmouth and for her severance package to be extended to six months.

The Portsmouth NAACP and the National Organization of Black Law Enforcement Executives planned to address Chapman's resignation at Tuesday's Portsmouth City Council meeting, according to NBC affiliate WAVY.

"We continue to witness patterns of blatant systemic racism resulting in the elimination of Black leaders in our City Governance and we are hell bent on fighting it through transparency, accountability and policy changes," the Portsmouth NAACP said in a statement after Chapman's resignation.