Within the heavenly city, Christ stands in the center, flanked by two archangels, the Virgin Mary, and St. Praxedes:
The twelve apostles approach from left and right. In this detail we see St. Paul and St. John the Baptist on the left, St. Peter and another apostle on the right. The other apostle is probably St. Andrew, given his untamed hair. Paul is characterized by his pointy beard and receding hairline, Peter by his short, square beard. John the Baptist wears a shaggy outer garment.
The elect on the left are led by Saints Praxedes and Pudentiana:
The arrival of the elect at the gate follows Revelation 22:14, "Blessed are they that wash their robes in the blood of the Lamb: that they may have a right to the tree of life, and may enter in by the gates into the city." For the angels at the gates, see Revelation 21:12, "it had a wall great and high, having twelve gates, and in the gates twelve angels." The elect bring golden crowns, like the "ancients" of Revelation 4:10, "The four and twenty ancients fell down before him that sitteth on the throne…and cast their crowns before the throne."
The first rank of these elect comprises the two sister saints (on the right in golden robes), a priest in green and a bishop in a red chasuble and long, white omophorion, and just behind them two deacons wearing golden dalmatics. All six present their golden crowns on hands covered by cloths. An archangel on the right gestures toward the gate of the Heavenly City, where another angel waits to greet them.
On the right side of the mosaic, the elect are led another bishop in red and priest in green.
The bishop and priest carry crowns while the man in white beside them carries a maquette of a church. He could be either Pope Hadrian I, who built the church in the 8th century, or Pope Paschal I, who commissioned these mosaics in the 9th. As on the left, an archangel gestures to the city, where another angel awaits. This archangel is flanked by St. Peter with his keys and St. Paul with a scroll.
Also see the apsidal arch, which continues these themes.
Read more about images of Saints Praxedes and Pudentiana.