The main liturgy at the Holy Mount of Grabarka, near the town of Siemiatycze, was officiated by Archbishop Abel in the absence of Archbishop Sawa, the head of the Polish Orthodox Church and Metropolitan of Warsaw and All Poland, state news agency PAP reported.
A group of central and local government officials also attended the ceremonies, alongside clergy from across the country.
This year's celebration coincided with the 1,700th anniversary of a religious event known as the Finding of the True Cross.
According to police estimates, about 6,000 people arrived at Grabarka for the two-day celebration, which began on Tuesday.
The Holy Mount of Grabarka, crowned by a small church and a convent, is a sacred site for Poland's Orthodox Christians.
The slopes of the hill are dotted with thousands of votive crosses placed there by pilgrims over the years.
Photo: PAP/Artur Reszko
The tradition of pilgrimages to Grabarka dates to the early 18th century when the site was believed to have saved people from a cholera epidemic.
Th Orthodox Church estimates that there are up to 500,000 Orthodox Christians in Poland.
The Feast of the Transfiguration is celebrated at the Holy Mount of Grabarka every year on August 18 and 19.
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Source: PAP