In Saudi Arabia, the United Arab Emirates and Indonesia, Saturday was declared the first day of fasting.
The start of Ramadan is determined by the sighting of the new crescent moon, which can vary by country.
In Iran, where Shia Islam is the state religion and the second-largest branch of Islam, Ramadan begins on Sunday.
During Ramadan, devout Muslims abstain from eating, drinking and smoking from dawn until sunset each day.
In the evenings, people traditionally gather for a meal to break their fast, known as iftar.
Fasting during Ramadan is one of the five pillars of Islam, which are the fundamental religious duties for Muslims.
After the fasting month, the multi-day celebration of Eid al-Fitr, also known as the "Festival of Breaking the Fast," follows.
There are approximately 1.9 billion Muslims worldwide, with the majority living in Indonesia, India, Pakistan and Bangladesh.
Muslims in Indonesia were shopping for sweets and new clothes and taking part in traditional festivities on Saturday.
Celebrations in the country range from colourful night-time parades and cleaning family graves to preparing food for pre-dawn breakfasts and elaborate post-sundown meals.
Each region in the vast archipelago of 17,000 islands has its own way to mark the start of Ramadan.
In the Middle East this year, the start of Ramadan coincides with the expiration of the first phase of a fragile ceasefire that had temporarily paused the 15-month-long war between Israel and the Palestinian militant organisation Hamas in the Gaza Strip.
Images showed residents shopping and preparing for Ramadan amid the ruins of the largely destroyed coastal strip.
Ramadan is considered a time of heightened tensions, particularly in the occupied Palestinian Territories and Jerusalem.
Hamas has called on its supporters to increase their presence at the al-Aqsa Mosque in Jerusalem during Ramadan.
The mosque is regarded as Islam's third holiest site after the al-Haram Mosque in Mecca and the Prophet's Mosque in Medina.
In Syria, the fasting month also began, marking the first Ramadan after decades of rule by the al-Assad clan in the war-torn country.
For the first time in 50 years, experts gathered on a mountain north of Damascus to observe the crescent moon and determine the beginning of Ramadan.
In many Middle Eastern countries, widespread concerns about the future cast a shadow over the start of Ramadan.
On Egyptian television, residents lamented economic hardship and high inflation.
In Lebanon - despite the current ceasefire between Israel and the Hezbollah militia - high prices and the consequences of the conflict dampened the mood.
with AP
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