U.S. Sen. Dianne Feinstein, the longest-serving woman senator in the country's history and the first woman to be San Francisco's mayor, has died at the age of 90, her office confirmed Friday.

"Senator Feinstein never backed away from a fight for what was just and right. At the same time, she was always willing to work with anyone, even those she disagreed with, if it meant bettering the lives of Californians or the betterment of our nation," James Sauls, Feinstein's chief of staff, said in a statement.

Feinstein, who was first elected to the U.S. Senate representing California in 1992, was a trailblazing politician -- the first woman to be president of the San Francisco Board of Supervisors who became mayor in 1978 after the assassination of Mayor George Moscone and Supervisor Harvey Milk.

The Democratic senator's legacy of legislation included the enactment of a federal assault weapons ban in 1994, the creation of the nationwide Amber Alert network for missing children, and the release of a report into the CIA's detention and interrogation practices that led to the passage of laws to ensure that some post-Sept. 11 interrogation methods would never be used again.

President Joe Biden called Feinstein "a true trailblazer and a cherished friend".

"In San Francisco, she showed enormous poise and courage in the wake of tragedy, and became a powerful voice for American values. Serving in the Senate together for more than 15 years, I had a front row seat to what Dianne was able to accomplish," the president said in a statement.

"Often the only woman in the room, Dianne was a role model for so many Americans -- a job she took seriously by mentoring countless public servants, many of whom now serve in my Administration. She had an immense impact on younger female leaders for whom she generously opened doors. Dianne was tough, sharp, always prepared, and never pulled a punch, but she was also a kind and loyal friend, and that's what (First Lady) Jill and I will miss the most," Biden said.

Gov. Gavin Newsom, himself a former San Francisco mayor, was among the local politicians to issue statements Friday morning mourning Feinstein's death.

"Dianne Feinstein was many things -- a powerful, trailblazing U.S. Senator; an early voice for gun control; a leader in times of tragedy and chaos. But to me, she was a dear friend, a lifelong mentor, and a role model not only for me, but to my wife and daughters for what a powerful, effective leader looks like," Newsom wrote.

Due to her death, Newsom is expected to appoint her replacement to the Senate. Feinstein said in February that she would not run for re-election. U.S. Rep. Adam Schiff (D-Burbank), Rep. Katie Porter (D-Irvine) and Rep. Barbara Lee (D-Oakland) are among the candidates running in 2024 for the seat.

Other local public officials issued statements in reaction to Feinstein's death on Friday.

U.S. Rep. Eric Swalwell (D-Livermore) called Feinstein "my friend and mentor ... a California icon."

"Her loss will be deeply felt across our state and our country," Swalwell said in a statement. "Senator Feinstein's dedication and leadership helped inspire me to get into public service. I'll never forget, after being elected to Congress, the Senator reached out to me to have lunch. She shared how important the work we do in Washington is and imparted upon me the most consequential part of our job: ensuring people feel safe in their communities."

"When I joined the Intelligence Committee, the senator was a terrific legislative partner as the Democratic leader of the committee in the Senate. Senator Feinstein and I are also bound by our friendship with the late Congresswoman, Ellen O. Tauscher, whom the Senator eulogized in 2019. The two were as close as sisters and represented the best of California," Swalwell added.

U.S. Senator Alex Padilla (D-California): "Dianne Feinstein was a towering figure not just in modern California politics, but in the history of our state and our nation ... She leaves behind a legacy of service, leadership, and a deep love for our country and our democratic ideals."

San Francisco Mayor London Breed: "My sympathies and prayers go out to her entire family and all San Franciscans who loved and admired her. Her dedication to public service was only exceeded by her love for San Francisco, and this City loved her back. Her legacy as one of San Francisco's true leaders will be admired for generations to come. Simply put, there will never be another Dianne Feinstein."

California State Senator Scott Wiener (D-San Francisco) issued the following statement in response to the passing of Feinstein:

"Dianne Feinstein was a true giant. She helped save our city, becoming Mayor after horrific political assassinations (and) leading us during the worst of the HIV/AIDS health disaster. As our Senator, she led on gun safety (and) so many issues.

"There will never be another Dianne Feinstein," he added.