The 15th-century astronomical clock in Prague's centre, which draws crowds of tourists every hour, was halted on Monday morning for six months of repairs, city hall said.

"The Old Town clock is one of Prague's symbols and its repair... is a necessary and responsible step," said a statement from Jan Wolf, the city's councillor for culture.

The clock, called Orloj, will be dismantled and taken away for repairs which will take five or six months.

The Old Town Hall, where the clock is installed, is "undergoing a complete reconstruction -- the first since World War II when most of the building was destroyed," said Wolf.

Every hour, crowds of tourists gather in front of the clock to watch figures of the twelve apostles that appear in two small oriel windows above a sophisticated astrolabe showing the movement of the Moon and the Sun and its entry into the zodiac signs.

Directed by a sculpture of Death pulling a bell cord, the apostles' parade takes about a minute until a gilded cock crows above the oriel windows.

The medieval clock's design is based on the theory that the Earth is the centre of the Universe and three quarters of its components are from the 15th-century original.

According to an old legend, Prague councillors blinded the clock's maker to prevent him from building another such device.

Prague, which welcomes about seven million tourists annually, might install a large screen featuring the clock and apostles while the clock is under repair, Wolf said.