It is being built by a businessman who said the idea came to him in a dream.
The Washington Post has reported that the construction follows a spate of religious attacks against Christians in the majority Muslim country.
Under Pakistan's blasphemy laws, anyone accused of insulting Islam, the Prophet Muhammad or other religious figures can face punishment.
Businessman Parvez Henry Gill said he had a dream two years ago in which God told him to do something for his community: "I want to show the world the Christian community in Pakistan has religious freedom."
He added that some people have criticised the cross but said: "I leave that to God."
Completion is expected to take a few months, the cross will stand at the entrance of a Christian cemetery in the centre of Karachi.
The cemetery was built almost 200 years ago under British rule.
Bishop Sadiq Danial of the Church of Pakistan, an Episcopal church, has offered to demolish the cross if it becomes a source of conflict between different faiths, but he added that he doubted that would happen: "We spread peace."