Does the Rapture mark the beginning of the Tribulation?
No, the Bible does not state anywhere that the Tribulation begins with the Rapture. I believe the Rapture could occur months or even years before the Tribulation begins, although it is likely to occur near the beginning because the Tribulation is the time of the pouring out of God's wrath, and 1 Thessalonians 1:10 says that Jesus will "deliver" His Church "from the wrath to come."
Another reason for believing the Rapture is likely to occur near the beginning of the Tribulation is because 2 Thessalonians 2 says that the Antichrist cannot be revealed until a "restrainer" is "taken out of the way" (2 Thessalonians 2:6-7). I believe that restrainer is the Holy Spirit working through the Church. Thus, when the Church is removed, the Antichrist will be unleashed, and the Tribulation will begin.
The prophet Daniel indicates that the starting point of the Tribulation will be a "covenant" that the Antichrist will arrange for Israel that will evidently guarantee the nation's peace and enable the Jews to rebuild their Temple (Daniel 9:27).
I want to address those who ask, "Aren't those of you who believe in a Pre-Tribulation Rapture a bunch of escapists who are not willing to suffer for the Lord?"
There is nothing wrong with being an "escapist." Noah was an escapist and so was Lot. And Jesus said that when the end time signs begin to appear, we are to pray "to escape the things that are about to take place and to stand before the Son of Man" (Luke 21:36).
Certainly we are called to suffer for Christ (Romans 8:17). And anyone who truly stands for Jesus in this world will be persecuted (John 15:19). We are assured that as believers we will suffer tribulation in this world (John 16:33), but we are promised that we will be exempted from the great tribulation that will one day come upon all the world (Revelation 3:10).