UNITED NATIONS (AP) _ A population clock at U.N. headquarters hit 6 billion today, marking the unprecedented global population growth that has seen the number of people on Earth double in less than 40 years.

The clock in the visitors lobby was racing so fast, in fact, that it skipped from 5,999,999,998 to 6,000,000,001.

According to U.N. demographers, the world's population reached 6 billion on Oct. 12.

The clock was placed inside the number 6 in a display set up by the U.N. Population Fund, which has advocated the right of women and men to determine the size and spacing of their families.

``It took all of time for world population to reach 2 billion in 1927 _ then less than a lifetime to arrive at 6 billion,'' the display said.

``Today there are 1.04 billion young people between 15 and 24 years of age ... Their choices will determine how many people will be on the planet by 2050 and beyond,'' it said.

Among the cameramen and photographers watching the clock tick were several visitors waiting to go on tours of the United Nations.

``I think we are too many people,'' said Natalia Sisinni, 26, an architecture student from Buenos Aires, Argentina. ``But I would not prefer to be alone, of course.''

``It was exciting,'' said Ron Kahan, who owns a marketing company in Denver, and joined the photographers in marking the symbolic milestone.

What about living among 6 billion? ``I just think it adds to the dynamic of the world, with the one Earth that we live in,'' he said.