(Reuters) - The Rev. Willie T. Barrow, a Chicago civil rights leader known as the "Little Warrior" for her small stature and determination to fight for equality, died on Thursday at age 90.

Barrow died at her Chicago home. She had recently been hospitalized for a blood clot in her lung, the Chicago Sun-Times reported.

Barrow had started her career in the civil rights movement when she was just 12 years old, demanding to be allowed to ride on an all-white school bus in her native Texas. She became an organizer for Dr. Martin Luther King Jr. and marched with him in Washington and in Selma, Alabama.

Barrow helped organize the Chicago chapter of Operation Breadbasket, which was dedicated to improving economic conditions in black communities, and she was important in persuading King to take his civil rights work to Chicago.

President Barack Obama, a former community organizer in the nation's third-largest city, said that for him and his wife, Michelle, Barrow was "a constant inspiration, a lifelong mentor, and a very dear friend."

"I was proud to count myself among the more than 100 men and women she called her 'Godchildren,' and worked hard to live up to her example," Obama said in a statement.

The Rev. Jesse Jackson Sr., head of the Rainbow PUSH Coalition civil rights organization which grew out of Operation Breadbasket, called Barrow an "authentic freedom fighter" in the lineage of Sojourner Truth, Rosa Parks and Fannie Lou Hamer.

"In sickness and death her body was frail, but her spirit and good works were never feeble," said Jackson. "Her flame of hope, freedom and justice will forever burn."

Jackson noted that Barrow had traveled to Nicaragua, Cuba, North Vietnam and Russia, and had been in South Africa when Nelson Mandela was released from prison.

Obama said Barrow had also stood up for labor rights and women's rights, welcomed gays and lesbians to the civil rights movement and had made one of the first pieces of the AIDS Memorial Quilt.

Chicago Mayor Rahm Emanuel ordered flags lowered at city facilities in Barrow's memory.