The BBC is under attack for using sound effects to mask the lyric "free Palestine" from a performance by rapper Mic Righteous on BBC Radio 1Xtra.

The corporation is being accused of bias after effectively editing the words from Mic Righteous's improvised set, in which he expressed his views on subjects ranging from the American government to poverty and the floods in Pakistan.

The song was recorded for Charlie Sloth's late night hip-hop show on 4 December, but protests have been mounting since the performance was rebroadcast as part of a "best of" from the show on 30 April.

The Palestine Solidarity Campaign on Wednesday posted a statement on its website urging listeners to write to the BBC and the Radio Times to complain, and to post their views on Sloth's 1Xtra home page.

Describing the edit as an "extraordinary act of censorship", the campaign asked why the BBC did not ban the song "Free Nelson Mandela" when it was released in 1984. At the time, Mandela was still considered a terrorist by many western governments.

A BBC spokesperson said a late night music show was not considered an appropriate forum for political controversy.

The corporation explained its decision in a statement: "All BBC programmes have a responsibility to be impartial when dealing with controversial subjects and an edit was made to Mic Righteous' freestyle to ensure that impartiality was maintained."

A video of the rap on YouTube has been played nearly 187,000 times.

Mic Righteous, who was raised in Margate, was recorded saying: "I can still scream 'Free Palestine' for my pride, still pray for peace, still burn the Fed for the brutality they spread over the world. Pakistan's a[sic] ocean of bodies in the brown water still nobody helps."

The number of comments on YouTube and Sloth's homepage is also increasing.

A critic calling themselves Speltwrong wrote: "Once again I find myself overwhelmingly disappointed by the blatant bias shown by the BBC towards the people of Palestine. Outrageous …"

John R Porter wrote: "Not content with banning an appeal by the Disasters Emergency Committee for humanitarian relief in Gaza, the wretched bureaucrats at the BBC have now decided that the very mention of Palestine in a song is unacceptable. The BBC deserves ridicule and contempt for this."