(Reuters) - Residents of Philadelphia should be ready for massive gridlock and prepared to walk for miles if they plan to travel downtown during Pope Francis' visit to the city in September, Mayor Michael Nutter said on Tuesday.
About 2 million people are expected to travel into the city for Francis' visit, which will feature an open-air Mass in front of the Philadelphia Museum of Art, as well as a separate week-long meeting organized by Roman Catholic leaders.
"This could in fact be the second or third largest event in the history of the United States. We're working to make sure that Philadelphia is open and accessible," Nutter told reporters, urging the city's 1.6 million residents to begin considering changes to their normal commutes and leaving their cars at home.
"Be prepared to walk at least a few miles or more," he said.
Local transit systems as well as Amtrak passenger trains will alter service around Philadelphia during the Pope's visit and special passes will be required to board, officials said.
The leader of the 1.2 billion-member church is also set to address the United Nations in New York and the U.S. Congress in Washington during his first visit to the United States that month.
The crowd at Philadelphia's Sunday Mass on Sept. 27 is expected to fill a 1.5 mile (2.4 km) stretch along the Schuylkill River, dwarfing the turnout for a 1979 event featuring Pope John Paul II and far eclipsing attendance seen at major sporting and holiday events, Nutter said.
"What you would normally do, to get to an Eagles game or something, forget that," Nutter said, referring to the city's National Football League franchise.
Francis's open, accepting tone has electrified U.S. Catholics, with liberal-leaning worshippers welcome his shift in focus away from issues including abortion and same-sex marriage, while conservatives have expressed dismay at his change in emphasis.
The visit is expected to give Philadelphia's economy a nearly $418 million boost, according to the Philadelphia Convention and Visitors Bureau.
Noting that Francis has drawn massive crowds at international events, including more than 6 million people in Manila in January that broke records for Papal travel, Nutter warned that Philadelphia's crowds could be larger than forecast.
"There is no specific, scientific, documentable way for us to actually figure out today, 90 days out, how many people will actually show up," he said.
(Reporting by Scott Malone in Boston; Editing by Susan Heavey)