Eid is the most important holiday on the Muslim calendar, and starts when the crescent moon is sighted.
Most Sunni Muslims began celebrations on Thursday, although most Shi'ites usually wait for an official announcement that the new moon has been sighted, marking the beginning of Shawal, which follows the fasting month of Ramadan.
Security was reinforced in Iraq on Thursday as Sunni Muslims and some Shi'ites celebrated the first of three days of Eid, which in past years has been marked by an increase in insurgent attacks.
Iraq's premier Shi'ite religious authority in Iraq, Grand Ayatollah Ali Sistani, decreed celebrations in his community are to begin Friday.
However supporters of radical Shi'ite cleric Moqtada Sadr ignored Sistani and have already made announcement proclaiming the start of the holiday in Sadr City.
The holiday season in recent years has been characterised by an increase in sectarian attacks on civilians.
At least 27 people were killed and 63 wounded on Wednesday when a minivan packed with explosives blew up in the Shiite town of Musayyib, at sundown outside a Shi'ite mosque.
And two other car bombs since Saturday have killed at least 44 people in markets in the southern city of Basra and the village of Huwaider, both Shi'ite communities.
In the West Bank, clashes left a Palestinian boy critically wounded by Israeli gunfire despite a call for calm from Palestinian leader Mahmoud Abbas at the start of Muslim holidays.
"The situation should be calm and all these clashes that have erupted here and there should stop," Mr Abbas told reporters after prayers in Gaza City.
Pakistan's President Pervez Musharraf has appealed to his nation to celebrate Eid with "simplicity", given the recent earthquake in Pakistan-controlled Kashmir last month that killed over 70,000 people.
"It is my appeal to you to remember the earthquake victims in your Eid festivities, celebrate Eid with simplicity" and donate money to victims, he said.
Security will be strengthened at the Faisal Mosque in Islamabad, and people have been advised to leave mobile phones, cameras and other electronic items at home.
India has also tightened security around mosques and religious shrines before the Muslim festival of Eid al-Fitr, as police pursue links to weekend bombings in New Delhi which they say point to Kashmir.
The number of police deployed at India's main mosque, the Jama Masjid in New Delhi's walled city, has been more than tripled from 30 to around 100, police inspector Darshan Singh told AFP.
Shoppers at a busy market have to pass through metal detectors, and police are also doing spot-checks on peddlers and shoppers.
Indonesia and Malaysia have marked the holiday in a more laidback manner.
Indonesia's president and other top officials opened the doors of their homes Thursday to welcome thousands celebrating Eid.
President Susilo Bambang Yudhoyono and First Lady Ani Yudhoyono shook hands with well-wishers who flocked to Negara Palace, where long queues of visitors were asked to leave their bags outside and pass through metal detectors before entering.
In Kuala Lumpur, Malaysian Prime Minister Abdullah Ahmad Badawi and Muslim cabinet ministers hosted tens of thousands of people in festivities on Thursday.
Around 150,000 Malaysians queued patiently at the Putra World Trade Centre in the city to greet Mr Abdullah and shake hands with cabinet ministers at the "open house" event.
They also tucked into festive dishes like beef rendang curry, fried noodles and cakes at the open house, a traditional practice in multi-ethnic Malaysia where people visit the homes of their Muslim friends.
Reflecting on the loss of his wife two weeks ago from breast cancer, Abdullah in his Eid address to the nation late on Wednesday urged Malaysians to strengthen family ties and remember departed loved ones, said the state Bernama news agency.