US President Donald Trump's administration has released a trove of records on the assassination of Martin Luther King Jr, including FBI surveillance files on the civil rights leader.

A court-imposed order had kept the documents, totalling 230,000 pages, blocked from public view since 1977.

Prominent members of King's family had opposed the release. A statement from his two living children condemned "any attempts to misuse these documents in ways intended to undermine our father's legacy".

King, a Baptist minister, was shot in Memphis on 4 April 1968, at age 39. James Earl Ray, a career criminal, pleaded guilty to the killing, but later renounced his plea.

King Jr's two living children, Martin III and Bernice, who were notified ahead of time about the release, said in a statement on Monday: "We ask those who engage with the release of these files to do so with empathy, restraint, and respect for our family's continuing grief.

"The release of these files must be viewed within their full historical context.

"During our father's lifetime, he was relentlessly targeted by an invasive, predatory, and deeply disturbing disinformation and surveillance campaign orchestrated by J Edgar Hoover through the Federal Bureau of Investigation."

The statement said the government's surveillance had denied King the "dignity and freedoms of private citizens".

The family also cited a jury verdict in a 1999 wrongful death civil lawsuit that found the civil rights leader was the victim not of a lone racist gunman, but of a vast conspiracy.

In January, Trump ordered that documents from the assassinations of King and former President John F Kennedy be declassified, along with the records in the assassination of Robert F Kennedy.

The Office of the Director of National Intelligence (DNI) said in a press release on Monday that the King files had "sat collecting dust in facilities across the federal government for decades, until today".

The documents include "internal FBI memos" and "never-before-seen CIA records" behind the hunt for King's assassin, the DNI said.

The release was coordinated with the FBI, Department of Justice, National Archives and CIA.

"The American people deserve answers decades after the horrific assassination of one of our nation's great leaders," US Attorney General Pamela Bondi said.

Not all of King's family was upset about the release.

Referring to the civil rights leader as "my uncle", Alveda King said: "I am grateful to President Trump and DNI Gabbard for delivering on their pledge of transparency.

"While we continue to mourn his death, the declassification and release of these documents are a historic step towards the truth that the American people deserve."

Trump's critics noted the release comes as the administration is accused of a lack of transparency over files relating to influential sex offender Jeffrey Epstein, whose 2019 jail death was ruled a suicide.

Civil rights leader Al Sharpton said the disclosure of the King files was "a desperate attempt to distract" from "the firestorm engulfing Trump over the Epstein files and the public unravelling of his credibility".

Before his arrest, King's convicted assassin, James Earl Ray, fled the country to Canada, Portugal and the UK, where he raided a bank.

Extradited to Memphis, he entered a guilty plea in 1969 and was sentenced to 99 years in prison.

He later claimed he had been framed by shadowy conspirators and tried to recant his guilty plea, but it was repeatedly upheld by the courts. Ray died in 1998 at 70.