US president Barack Obama issued a challenge to world leaders and ordinary people to live up to the values championed by former president Nelson Mandela.

"We, too, must act on behalf of justice. We, too, must act on behalf of peace," Obama said in his tribute at a memorial service for Mandela at the FNB Stadium in Soweto on Tuesday.

"There are too many of us who happily embrace Madiba's legacy of racial reconciliation, but passionately...read more here: Live Mandela legacy: Obama