Deadly bombings cut short New Year celebrations in Bangkok and an ETA bombing prompted Madrid to cancel its festivities, while bad weather hampered revellers from New Zealand to Scotland.
But the capitals of Bulgaria and Romania saw their biggest parties since the fall of Communism 17 years ago as tens of thousands sang, danced and drank their way into 2007 and the European Union.
Spectacular sound and light shows lit up Sofia and Bucharest as the two countries celebrated becoming the 26th and 27th members of the bloc, the sky exploding with fireworks in EU blue and gold at the stroke of midnight.
In the Thai capital Bangkok, two people were killed and more than 30 injured, including foreigners, in a mysterious wave of blasts which forced the ruling military junta to call off official events.
Authorities in Madrid also cancelled a traditional New Year fiesta in the city centre as a precaution following Saturday's airport bomb blast claimed by the armed Basque separatist group ETA.
The blast injured 19 people and two Ecuadorians remains missing. If confirmed dead, they would be the first fatalities ascribed to ETA since 2003.
Security was tight in the Indian capital New Delhi and police arrested two Islamic militants carrying explosives hidden in toys which they had planned to detonate at a busy market.
Meanwhile, severe weather forced the cancellation of festivities in New Zealand and northern parts of Britain.
An annual outdoor New Year's Eve party in the New Zealand capital Wellington was cancelled due to rain and some of the coldest summer temperatures on record.
Celebrations in the Northern Irish capital of Belfast, the Scottish hubs of Edinburgh and Glasgow as well as the northern English cities of Newcastle, Liverpool and Leeds were all canceled over weather-related safety fears.
Up to 100,000 people had been expected to join the annual Hogmanay (last day of the year) celebration in Edinburgh, the focal point for Scotland's biggest holiday.
But winds of up to 70 miles per hour (110 kilometers per hour) made temporary stages unsafe, prompting cancelation of pop acts like the Pet Shop Boys and Paolo Nutini as well as a fireworks display.
However the vast majority of revellers around the world celebrated peacefully, with an estimated one million people thronging Australia's Sydney Harbour for a huge fireworks show.
Sydney police said all vantage points around the shoreline were at or near capacity as some 3,000 kilograms (6,600 pounds) of fireworks exploded overhead.
In Hong Kong, 2007 brought immediate changes as midnight heralded sweeping new anti-smoking laws that technically made puffing party-goers liable to an on-the-spot-fine.
Hundreds of thousands of revellers descended on the downtown Lan Kwai Fong bar area and along the harbourside to watch the annual midnight fireworks display. Last year's fireworks attracted more than half a million people.
In London, an estimated 150,000 lining the River Thames cheered at the dazzling and deafening 15-minute fireworkes display over the Houses of Parliament's famous clock tower as well as the London Eye, the world's tallest observation wheel.
In Paris, an estimated 400,000 crowded the Champs-Elysees to ring in the New Year. In another New Year tradition, French youths had torched around 100 cars throughout the country before midnight, police said.
Taking the French love to say "non" to a new extreme, some 600 people gathered in the western city of Nantes not to ring in the New Year, but to protest its arrival.
Lashed by rain, the organisers joked even the weather was against 2007, as they milled about under banners reading "No to 2007!" and "Now is better!"
Germans took advantage of mild weather, with organisers saying one million people attended festivities at the capital's Brandenburg Gate.
In Moscow, revellers were reportedly turned away from Red Square once a capacity 15,000 crowd filled the square for a midnight concert and firework display.
An artificial ice rink set up on the square charged a special price of 2007 rubles (76 dollars, 58 euros) for two hours and seven minutes of skating.
The New Year coincides with the Muslim festival of Eid al-Adha, creating the opportunity for a double celebration in Muslim-majority countries.
But the party mood quickly dissipated for more than 1,000 Turks who had to seek hospital treatment on Sunday after their knives slipped badly as they held down struggling goats, sheep and bulls during attempts at ritual slaughter at the start of the Muslim Feast of the Sacrifice.
Across the Atlantic, a vast crowd was expected in Times Square in New York at the end of a tumultuous year for the United States over the war in Iraq.