Countries around the globe marked the world's population reaching seven billion on Monday with lavish ceremonies for newborn infants symbolizing the milestone and warnings that there may be too many humans for the planet's resources.

While demographers are unsure exactly when the world's population will reach the seven-billion mark, the United Nations is using Monday to symbolically mark the day. A string of festivities are being held worldwide on Monday, with a series of symbolic seven-billionth babies being born.

Motorists wait at a traffic junction in Hanoi, Vietnam, on Monday. Kham/ReutersThe celebrations began in the Philippines, where baby Danica May Camacho was greeted with cheers and an explosion of photographers' flashbulbs at Manila's Jose Fabella Memorial Hospital. She arrived two minutes before midnight Sunday, but doctors say that was close enough to count for a Monday birthday.

The baby received a shower of gifts, from a chocolate cake marked "7B Philippines" to a gift certificate for shoes.

"She looks so lovely," the mother, Camille Galura, whispered as she cradled the 5.5-pound baby, who was born about a month premature.

The baby was the second for Galura and her partner, Florante Camacho, a struggling driver who supports the family on a tiny salary.

Dr. Eric Tayag of the Philippines' Department of Health said later that the birth came with a warning.

"Seven billion is a number we should think about deeply," he said.

"We should really focus on the question of whether there will be food, clean water, shelter, education and a decent life for every child," he said. "If the answer is no, it would be better for people to look at easing this population explosion."

In New York, UN Secretary-General Ban Ki-moon delivered an eloquent speech that also noted the sharp disparities among the world's inhabitants.

"Our world is one of terrible contradictions," he said. "Plenty of food but one billion people go hungry. Lavish lifestyles for a few, but poverty for too many others."

1 billion reached in 1804

Demographers say it took until 1804 for the world to reach its first billion people, and a century more until it hit two billion in 1927. The 20th century, though, saw things begin to cascade: three billion in 1959; four billion in 1974; five billion in 1987; six billion in 1998.

The UN estimates the world's population will reach eight billion by 2025 and 10 billion by 2083. But the numbers could vary widely, depending on everything from life expectancy to access to birth control to infant mortality rates.

In Uttar Pradesh, India — the most populous state in the world's second-most populous country — officials said Monday they would be appointing seven girls born Monday to symbolize the seven billion.

India, which struggles with a deeply held preference for sons and a skewed sex ratio because of millions of aborted female fetuses, is using the day to highlight that issue.

"It would be a fitting moment if the seven billionth baby is a girl born in rural India," said Dr. Madhu Gupta, an Uttar Pradesh gynecologist. "It would help in bringing the global focus back on girls, who are subject to inequality and bias."

China stands by one-child policy

Meanwhile China, which at 1.34 billion people is the world's most populous nation, said it would stand by its one-child policy, a set of restrictions launched three decades ago limiting most urban families to one child and most rural families to two.

"Overpopulation remains one of the major challenges to social and economic development," Li Bin, director of the State Population and Family Planning Commission, told the official Xinhua News Agency. He said the population of China would hit 1.45 billion in 2020.

While the Beijing government says its strict family planning policy has helped propel the country's rapidly growing economy, it has also brought many problems. Soon, demographers say, there won't be enough young Chinese to support its enormous elderly population.

China, like India, also has a highly skewed sex ratio, with aid groups saying sex-selective abortions have resulted in an estimated 43 million fewer girls than there should be, given the overall population.

India, with 1.2 billion people, is expected to overtake China around 2030 when the Indian population reaches an estimated 1.6 billion.