Wedged in between City Hall and the Old Post Office building with the Feather Market Centre on the other side of the right is the Cross of Prester John. The monument has no significant link to Port Elizabeth other than the fact that it was a stop en route to the East for Portuguese explorers who, in addition to looking for a way around Africa to the East, were also hoping to make contact with Prester John as a Christian ally. A local philanthropist paid for the monument which was unveiled by the Portuguese Ambassador to South Africa in 1986.
The story of Prester John is a mysterious one. In some circles he was believed to be a descendant of the Three Wise Men, some believed he was a crusader-era Christian king based in Ethiopia or possibly a high-born Mongol from the time of Genghis Khan. Then there were those who said that he watched over the Holy Grail, never growing old but wiser and wiser as the years went by. Whoever this mythical king-priest Prester John was, it was the quest of the Portuguese explorers not just to find a sea route around Africa to the East, but to also find and make contact with Prester John as a Christian ally.
Prester John (Latin: Presbyter Johannes) is a legendary Christian patriarch and king popular in European chronicles and tradition from the 12th through the 17th centuries. He was said to rule over a Nestorian (Church of the East) Christian nation lost amid the Muslims and pagans of the Orient, in which the Patriarch of the Saint Thomas Christians resided. The accounts are varied collections of medieval popular fantasy, depicting Prester John as a descendant of the Three Magi, ruling a kingdom full of riches, marvels, and strange creatures.
At first, Prester John was imagined to reside in India; tales of the Nestorian Christians' evangelistic success there and of Thomas the Apostle's subcontinental travels as documented in works like the Acts of Thomas probably provided the first seeds of the legend. After the coming of the Mongols to the Western world, accounts placed the king in Central Asia, and eventually Portuguese explorers convinced themselves that they had found him in Ethiopia.
You can read more about Prester John on Wikipedia where I got the above information.