Thousands of celebrants gathered at Stonehenge in England on Sunday to mark the summer solstice at the ancient Neolithic standing stone site.

Police estimated the crowd gathered to celebrate the longest day in the Earth's Northern Hemisphere at around 23,000, less than the 30,000 expected and far below the 36,000 who congregated at the ancient site last year.

Although the crowd gathered at the site on the Salisbury Plain around 90 miles southwest of London was able to witness the sunrise at 4:52 a.m., to the accompaniment of drums played by many of the assembled celebrants, clouds soon moved in, cutting the party short for many.

Police reported just nine arrests, for drug offenses, down from past years.

"The success of the event depends largely on the good nature of those attending and we are pleased that people could enjoy solstice in the spirit of the event," said Wiltshire Police Superintendent Gavin Williams.

Stonehenge, built in phases from 3,000 B.C. to 1,600 B.C., is one of Britain's most popular tourist attractions, with millions of visitors every year.

The ring of standing stones, set within a series of earthworks, is believed by many archaeologists to have been a ceremonial burial ground from its earliest days.

Stonehenge sits in the center of a an extensive complex of Neolithic and Bronze Age monuments and sites, surrounded by several hundred burial mounds.

It is believed to have been an important religious site as far back as 4,000 years.

Stonehenge was added to UNESCO's list of World Heritage Sites in 1986.

Alignments of some of the stones and other features linked to Stonehenge suggest it was linked to ceremonies marking both the midwinter and summer solstices, experts have proposed.

Self-styled Druids and so-called New Agers have gathered on the site for years to mark such occasions, particularly the summer solstice.

The summer solstice, which brings the day with the longest period of daylight and is considered the official start of summer, is when the sun's path reaches the farthest north as the Earth's North Pole reaches its maximum tilt toward the sun.

In the Southern Hemisphere, which at the moment has its pole tilted the farthest away from the sun, the summer solstice occurs sometime between December 20 - 23.