About 1000 people have gathered to commemorate the 20th anniversary of the Oklahoma City bombing, the deadliest terrorist attack on US soil until the September 11 attacks six years later.

Former President Bill Clinton and Oklahoma Governor Mary Fallin were among those who spoke at the service at the Oklahoma City National Memorial, where the Alfred P Murrah Federal Building once stood.

The service started with 168-seconds of silence to honour each of the victims who died in the attack and concluded about 90 minutes later with survivors and tearful relatives of the dead reading the names of those killed.

Timothy McVeigh, an Army veteran with strong anti-government views, was convicted on federal murder and conspiracy charges in 1997 and executed in 2001.