NEW YORK (AFP) - Rap mogul Jay Z has quietly used his wealth to post bail for people arrested in protests across the United States against the police, an author close to him said on Sunday.

Dream Hampton, a writer and activist who worked with Jay Z on his 2010 memoir Decoded, made the revelations in a series of messages on Twitter that were deleted but were posted by the hip-hop magazine Complex.

"When we needed money for bail for Baltimore protesters, I...hit Jay up, as I had for Ferguson" and he "wired tens of thousands" of dollars within minutes, read one tweet.

She also tweeted that Jay Z and his pop superstar wife Beyonce wrote a "huge cheque" to support the burgeoning Black Lives Matter movement aimed at stopping police brutality.

Protests spread last year after a white police officer shot dead African-American teenager Michael Brown in the St. Louis suburb of Ferguson.

Major demonstrations, some turning violent, erupted last month in Baltimore after another African American man, Freddie Gray, died from a spinal injury sustained in police custody.

The tweets appeared aimed at defending Jay Z and Beyonce, who have faced accusations from some activists that they have only paid lip service to the cause without tapping into the couple's estimated US$1 billion (S$1.3 billion) net worth.

Hampton later wrote that she deleted the tweets because Jay Z "would be pi-issed to see I was offering evidence" that he is taking action.

Jay Z on Saturday performed in New York as an exclusive for Tidal, the music streaming service he has launched with fellow stars.

In a freestyle segment, he paid homage to Brown and Gray - and belittled Tidal's rivals Spotify and Apple.

Jay Z and Beyonce met Brown and Gray's families last week at a peace concert in Baltimore by Prince, leaving without making public appearances.

The power couple are also strong supporters of President Barack Obama, throwing a multimillion-dollar fundraiser for him during his 2012 campaign.