Muhammad Ali -- 'The Greatest' Arrives in London for Olympics

Muhammad Ali ... the GREATEST of All Time ... has arrived in London for the 2012 Summer Games ... 52 years after winning his own gold medal.

Ali was spotted leaving his hotel Tuesday with some help from his wife Lonnie (left).

There are rumors Ali will have a role in the opening ceremonies on Friday ... but so far, no one has confirmed one way or the other.