Question:  Leonard "Len" Benker Johnson organized a concert and a rally featuring an American bass singer who became involved with what?

<P0-S0>  Johnson (who was of mixed race) was denied the opportunity to fight for championships in Great Britain because of the colour bar that existed in boxing at that time. <P0-S1>  He was managed by his father Bill Johnson. <P0-S2>  Johnson fought on boxing booths in the 1920s, and from 1930 to 1939 owned his own travelling booth. <P0-S3> Leonard "Len" Benker Johnson (22 October 1902 – 28 September 1974 (aged 71)) born in Clayton, Manchester was an English professional middle/light heavyweight boxer of the 1920s, and 1930s, who won the Australian version of the British Empire middleweight title in 1926. <P0-S4>  Just prior to the outbreak of World War Two Johnson briefly tried his hand at professional wrestling, but once war came he lost interest in both boxing and wrestling, and sold his booth. <P0-S5>  During World War Two Johnson was a member of the Civil Defence heavy Rescue Squad in Manchester. <P0-S6>  Len Johnson was also a member of the Communist Party of Great Britain (joining the party in 1944 and later writing a boxing column for the party newspaper "The Daily Worker"), a trade unionist, a community leader in Moss Side, a local civil rights activist, and attended the 5th Pan-African Congress in Manchester. <P0-S7>  Following the Second World War, he was a co-founder of the New International Society in Manchester, which provided a vehicle for black political and social self-organisation in the city and campaigning against racism at home and abroad, including organising a concert and a rally featuring Paul Robeson in support of the Trenton Six, which were both attended by 10,000 people. <SEP> <P1-S0> Richard Anthony Sterban (born April 24, 1943) is an American bass singer born in Camden, New Jersey, who joined the country and gospel quartet The Oak Ridge Boys in 1972. <P1-S1>  Prior to joining The Oak Ridge Boys, Sterban toured with J. D. Sumner and the Stamps Quartet, who were singing backup for Elvis Presley at that time. <P1-S2>  Sterban ultimately became best known for his "oom-pa-pa-oom-pa-pa-mow-mow" bass solo in the Oak Ridge Boys' 1981 single "Elvira" and sang lead vocals on a select few of the group's songs, including a cover of The Righteous Brothers' hit "Dream On," which itself became a hit. <SEP> <P2-S0> Larry Gene Ashbrook (July 10, 1952 – September 15, 1999) was an American mass murderer who killed seven people and injured a further seven at a post See You at the Pole Rally featuring a concert by a Christian rock group Forty Days before committing suicide. <P2-S1>  It occurred at Wedgwood Baptist Church in Fort Worth, Texas, on September 15, 1999. <SEP> <P3-S0>  At Rutgers College, he was an outstanding American football player, and then had an international career in singing, with a distinctive, powerful, deep bass voice, as well as acting in theater and movies. <P3-S1>  He became politically involved in response to the Spanish Civil War, fascism, and social injustices. <P3-S2> Paul Leroy Robeson (pronounced ; April 9, 1898 – January 23, 1976) was an American bass singer and actor who became involved with the Civil Rights Movement. <P3-S3>  His advocacy of anti-imperialism, affiliation with communism, and criticism of the United States government caused him to be blacklisted during the McCarthy era.