The UN has appealed for much greater investment in family planning to help reduce poverty and slow down population growth.
Marking World Population Day, UN Secretary-General Ban Ki-Moon said the benefits of family planning remained out of reach of many people, especially the poor.
The UN estimates that the world's population will grow by more than a third by the year 2050 - from 6.7bn now to 9.2bn - a growth rate it says is unsustainable.
More than 140 countries worldwide will observe World Population Day today by emphasising the importance of family planning for the well-being of families, communities and nations, and by underlining the need to further integrate such services into national development plans.
The theme of World Population Day 2008 is 'Family Planning: It’s a Right; Let’s Make it Real.'
It provides a chance to raise awareness of the benefits of family planning, including its role in enhancing maternal health, gender equality and poverty reduction.
World leaders have proclaimed that individuals have a basic human right to determine freely and responsibly the number and spacing of their children, yet modern contraception remains out of reach for hundreds of millions of women and men.
World Population Day activities range from rallies, performances and exhibitions to sports competitions, seminars and cultural events.
In his World Population Day message, Ban Ki-moon said 'Studies show that family planning has immediate benefits for the lives and health of mothers and their infants.'
'Let us take action to reduce maternal mortality and achieve universal access to reproductive health by 2015,' said Mr Ban.
In a separate message, Thoraya Ahmed Obaid, Executive Director of UNFPA, the UN's Fund for Population Activities, said family planning 'is essential to women’s empowerment and gender equality. When a woman can plan her family, she can plan the rest of her life.'
