By state law, ballots received by mail cannot be opened and counted until Election Day.
Officials will begin processing and scanning signatures when polls open at 7 a.m. Tuesday.
City leaders said that means it could easily take several days to get a tally of the mail-in ballots.
State election officials said the goal is to have every vote counted by Friday or earlier across Pennsylvania.
READ ALSO: Philadelphia prepares for 'avalanche' of mail-in ballots
Philadelphia Mayor Jim Kenney and Lisa Deeley, chair of the Philadelphia City Commissioners, asked for patience in an open letter to Philadelphians.
They and said election officials will focus on a few election concepts: fairness, integrity, access and accuracy.
Kenney and Deeley said the process will not favor any party or group and that aggressive behavior at polls will not be tolerated.
RELATED: 'Voter Express' trolleys transport voters to the polls in Philadelphia
Election officials will ensure that no ballots will be messed with or tainted. They said voting machines have been tested in two prior elections and that there will be well-trained staff and polling volunteers.
The city is opening 718 voting locations, according to Kenney and Deeley. The two said nearly 85% of voters will be voting in the polling location they have used in their neighborhoods for years.
City leaders stress that turnout will be strong, so expect long lines at the polls.
Lines will seem even longer because of the need for everyone to stay six feet apart.
Polls are open from 7 a.m. to 8 p.m.
If you are in line by 8 p.m., you must be allowed to vote under state law.
SEE ALSO: Vote 2020: Polling locations and more Election Day info for Pa., N.J. and Del.
The full text of the letter issued by the mayor's office can be found below:
Amid all the speeches and slogans, the tweets and talk shows, the rallies and rhetoric, the election in Philadelphia really comes down to some very simple concepts: fairness, integrity, access, and accurate results.
We are writing to ask that as you prepare for tomorrow's election, remember those four principles. They are not empty words or phrases. They are the core of what we are trying to accomplish.
Fairness: The City's election planning, processes and procedures favor no party or group. Our bipartisan City Commissioners have stressed that at every step. This will be a truly democratic election -- small 'd' -- and no voter should face intimidation or coercion by anyone. Any aggressive behavior or voter intimidation at the polls won't be tolerated -- this includes attempts by individuals who are not certified poll watchers to access polling locations and observe voting.
Members of the public are encouraged to report possible criminal attempts to interfere with or influence voting and election activity by calling the DAO Election Task Force hotline at 215-686-9641.
Integrity: The voting -- in-person on Tuesday or mail-in ballots submitted early -- will not in any way be manipulated or tainted. The voting machines that are in place for November 3 have been tested now in two prior elections.This prior use means that well-trained staff and polling place volunteers have the experience to ensure the machines are set up correctly and will be operating properly. And our operation to count mail-in ballots has added more equipment and staff to help speed the process.
Access: This election, we're opening 718 voting locations -- more than triple the number of polling places available in the June primary. That means nearly 85 percent of voters will be voting in the polling location they have used in their neighborhoods for years. All information is offered in both English and Spanish, interpretation services are provided at every polling place, COVID safety precautions are in place, and all locations are accessible for people with disabilities.
Accurate results: Our election officials will be working tirelessly to count and double-check every ballot, They have worked closely with the firm that manufactures the voting machines to ensure accurate tallies. The Commissioners have set up a tremendous, secure facility at the Convention Center to process and count mail-in and absentee ballots -- more than 400,000 at last count.
There's a fifth concept that we need you to contribute: patience.
We expect long lines at the polls. Not only will turnout be strong, but the importance of the choices on the ballot means that everyone will be taking their time before locking in their vote. And the lines will seem even longer because of the need for everyone to stay six feet apart.
So on Election Day, please be patient -- the people working your voting station and waiting with you are your neighbors. They are doing their best. And remember that so long as you are in line by the time the polls close at 8 p.m., you will be able to vote.
After the polls close, and in the ensuing days, we will continue to need your patience. Never in the history of this city have so many people voted by mail. By law, staffers are not allowed to start opening and counting these ballots until Election Day itself.
That means getting a tally of mail-in ballots will easily take several days. This may determine the outcome in Philadelphia, and in the Commonwealth as a whole. So, again, please be patient.
Patience brings calm. We know all Philadelphians have been through a difficult year, and you know the importance of this election. So we close this letter with one last ask: if you should encounter frustrating or difficult situations on Election Day, let your inner strength guide you. Stay calm, stay respectful, stay above the fray.
With your help, we are confident that our city will shine as an example of how to run an election right.
Because we are Philadelphia.