Police in Alabama arrested six African-American civil rights activists staging a sit-in at Senator Jeff Sessions' office on Tuesday to protest his nomination for US Attorney General, criticising his record on voting rights and race relations.

Sessions, 70, has a history of controversial positions on race, immigration and criminal justice reform.

Members of the National Association for the Advancement of Colored People (NAACP) had vowed to occupy Sessions' Mobile, Alabama office until the conservative Republican lawmaker either withdrew as a candidate or they were arrested.

In the end, Cornell Brooks, president and CEO of the NAACP, and Stephen Green, national director of the youth division of the NAACP, were among those arrested, according to a post on the Twitter page of the civil rights organisation.

The other four protesters arrested by police included Benard Simelton, president of the NAACP's Alabama state conference, and Devon Crawford of the group Humanity in Action, according to local news outlet AL.com.

A spokesman for Mobile police could not be reached for comment late on Tuesday.

"Senator Sessions has callously ignored the reality of voter suppression but zealously prosecuted innocent civil rights leaders on trumped-up charges of voter fraud," Brooks said in a news release before his arrest.

Brooks had posted a photo on Twitter of protesters in suits occupying the senator's Mobile office.

A spokeswoman for Sessions called the NAACP's criticisms "false portrayals."

"Jeff Sessions has dedicated his career to upholding the rule of law, ensuring public safety and prosecuting government corruption," spokeswoman Sarah Isgur Flores said in a statement. "Many African-American leaders who've known him for decades attest to this and have welcomed his nomination to be the next Attorney General."

President-elect Donald Trump in November named Sessions to lead the Justice Department and the FBI, and his history could come under scrutiny from fellow senators during a confirmation process.

Sessions was a US prosecutor in 1986 when he became only the second nominee in 50 years to be denied confirmation as a federal judge. This came after allegations that he made racist remarks, including testimony that he called an African-American prosecutor "boy," an allegation Sessions denied.

Sessions denied he was a racist and said at his hearing that groups such as the NAACP and the American Civil Liberties Union could be considered "un-American."

He also acknowledged calling the Voting Rights Act of 1965 "intrusive legislation."