Muslim religious leaders have declared Friday as the start of Ramadan in much of the Arab world.

The start of the holiest month in the Islamic calendar is marked by the sighting of the new moon, which varies from country to country.

During Ramadan, Muslims are expected to fast and abstain from smoking, sex and drink from sunup to sunset, as well as to perform good deeds and acts of charity.

The month marks the time more than 1,400 years ago when Muslims say the Prophet Muhammad first received the words of the Quran — the Muslim holy book.

Mainly Muslim Indonesia and Muslims in China also will celebrate Ramadan on Friday.