Officials plan to construct an 8-foot tall fence to enclose a more than 4 mile stretch of Center City for Pope Francis' visit in September, multiple sources with knowledge of security plans tell NBC10.

The perimeter fence will start to be erected on Monday, September 21 and cover a 4-and-a-half mile swath of downtown. The area will then be swept by security personnel and go through rolling security checks through the weekend, sources said.

On Friday, September 25, the day before the pope arrives, sources said a no vehicle zone will be established on the outside of the fence stretching up to four blocks away.

More than 1.5 million people are expected to come to Philadelphia to see Pope Francis and take part in a Sunday mass along the Benjamin Franklin Parkway. It will be his first visit to the United States since becoming the Roman Catholic church's leader. The pope will make eight public appearances while in the city over a two-day span September 26 and 27.

The U.S. Secret Service is the lead agency on security for the papal visit. A spokesman for the mayor's office said the city is not currently involved in a fencing plan. A Secret Service spokesman declined comment.

It's not clear if roads will remain open inside the fence before Friday. City officials said they were not ready to release road closure details at a briefing last month about mass transit operations during the pontiff's visit.

Mayor Michael Nutter did say visitors and residents should "be prepared to walk for miles" during Pope Francis' time in the city.