PTI has planned a rally at Mazar-e-Quaid, to be addressed by PTI chief Imran Khan in the evening.
The interior ministry said terrorist threat existed at the sit-in venue as well as other important parts of the city.
It asked the Sindh government to adopt stringent security measures across the city. The letter asked the government to enhance security across all public places and sensitive installations.
PTI chairman Imran Khan will be landing in Karachi around 5pm to address the mammoth public gathering in Karachi. He will leave the city at 8.30pm under tentative plans as elaborated by the PTI.