JOHANNESBURG — U.S. President Barack Obama exhorted the world Tuesday to embrace Nelson Mandela’s universal message of peace and justice, electrifying tens of thousands of rain-lashed spectators and prompting a standing ovation by scores of heads of state in a South African stadium.

In a speech that received thunderous applause, Obama urged people to apply the lessons of Mandela, who emerged from 27 years in prison under a racist regime, embraced his enemies when he finally walked to freedom and promoted forgiveness and reconciliation in South Africa.

“We, too, must act on behalf of justice. We, too, must act on behalf of peace,” said Obama, who like Mandela became the first black president of his country. Obama said that when he was a student, Mandela “woke me up to my responsibilities — to others, and to myself — and set me on an improbable journey that finds me here today.”