World Population Day is observed annually on 11 July to raise awareness of global population issues. The date was chosen because it was on or around 11 July 1987 that the world's population reached five billion people. That figure is now nearly 7.5 billion, and the United Nations estimates it will be around 11.2 billion by the year 2100. Part of the problem is that people are living longer – life expectancy at birth increased from 30 to 67 between 1800 and 2005.

China is the world's most populous country with around 1.4 billion inhabitants, but its population is falling, and India is catching up rapidly. This year's theme is "investing in teenage girls" who face a lack of school and early marriage and motherhood which leaves them vulnerable to illness, injury and exploitation.

The growth of the world's population affects sustainability, urbanisation, health, inequality, poverty and global warming. To mark World Population Day 2016, IBTimes UK presents this gallery of overcrowding and huge gatherings around the globe.

World Population Day 2015
People crowd onto a bridge that they believe can help them escape tribulations and illnesses during the annual Caiqiaohui event in Mianyang, Sichuan province, China Reuters
World Population Day 2015
An artificial wave pool is packed by would be swimmers to escape the summer heat in Daying county in Sichuan province, ChinaChina Daily/Reuters
World Population Day
People scramble to reach the roof of an overcrowded train at a station in Dhaka, during the annual festival of Eid al-Adha in 2015Roberto Schmidt/AFP
World Population Day 2015
Scooters and motorcycles at a junction during rush hour in Taipei, Taiwan, in 2009Nicky Loh/Reuters
World Population Day 2015
People try to touch the religious icon of the Black Nazarene during the annual procession in the Philippines capital Manila on 9 January 2015 attended by over 5 million peopleTed Aljibe/AFP
World Population Day
Devotees try to form a human pyramid during celebrations to mark the Hindu festival of Janmashtami in Mumbai, IndiaDanish Siddiqui/Reuters
World Population Day
Students practice tai chi on a playground at a high school in Jiaozuo, Henan province, China, during a Guinness World Record attempt for the largest martial arts display, on 18 October 2015Reuters
World Population Day
A traffic jam near a toll station on a huge highway as people return home at the end of a week-long national day holiday, in Beijing, China, on 6 October 2015China Daily/Reuters
World Population Day
Commuters disembark from crowded suburban trains during the morning rush hour at Churchgate railway station in Mumbai, IndiaVivek Prakash/Reuters
World Population Day
Jobseekers visit booths of companies at a job fair for graduates in Nanjing, Jiangsu province, ChinaSean Yong/Reuters
World Population Day 2015
Applicants walk into the entrance of a university to take part in the entrance exam for postgraduate studies in Wuhan, Hubei province, ChinaReuters
World Population Day 2015
Shi'ite Muslims gather at the shrine of Imam Hussein during Ashura commemorations in Karbala, Iraq, on 4 November 2014 when an estimated 20 million people visited the holy siteMohammed Sawaf/AFP
World Population Day 2015
Muslim pilgrims pray at the Grand Mosque in the holy city of Mecca during the annual hajj pilgrimageFile photo Ibraheem Abu Mustafa/Reuters
World Population Day
Lifeguards attempt to control the crowd as people visit the beach on new year's day in Durban, South Africa, on 1 January 2014Rogan Ward/Reuters
World Population Day
A crowded beach in Durres, on the Albanian AdriaticAFP
World Population Day
Couples pose for a group photograph during a mass wedding ceremony at the Taipei Flora Expo Hall in Taiwan on 9 September 2010Nicky Loh/Reuters
World Population Day
Commuters are held outside Oxford Circus tube station in London, United Kingdom, to regulate numbersMatthew Lloyd/Getty Images
World Population Day 2015
Buses and commuters cross the diagonal crossing, aimed at easing crowding, at Oxford Circus in London, United KingdomStefan Wermuth/Reuters
World Population Day 2015
Commuters wait for a train at a subway station in Sao Paulo, BrazilNacho Doce/Reuters
World Population Day 2015
Passengers crowd into a departure hall while waiting to board trains at a railway station in Wuhan, in China's Hubei province, during the annual new year mass migrationReuters
World Population Day
People walk along a crowed shopping street on the first day of the winter sales in Barcelona, SpainDavid Ramos/Getty Images
World Population Day 2015
Staff push a passenger into a crowded subway train at the Ikebukuro station on the Marunouchi line during rush hour in Tokyo, the Japanese capitalKim Kyung Hoon/Reuters
World Population Day 2015
A passenger uses a window to board a crowded train at a railway station in Hefei, Anhui province, during China's new year travel peak season Jianan Yu/Reuters
World Population Day 2015
Laundry hangs outside a student dormitory at a college in Wuhan, Hubei province, ChinaReuters
World Population Day 2015
Residential buildings in Po Lam, one of Hong Kong's satellite towns. About 25 percent of the world's tallest 100 residential buildings, over 200 metres tall, are in the territoryBobby Yip/Reuters
World Population Day 2015
Thousands of internally displaced people queue for the distribution of goods at Kibati camp near the city of Goma in the eastern Democratic Republic of Congo on 10 December 2008Peter Andrews/Reuters
World Population Day 2015
An overcrowded train leaves a platform in Tongi near Bangladesh's capital Dhaka in 2003, as pilgrims flock to the annual Ijtema Muslim gatheringRafiquar Rahman/Reuters
World Population Day 2015
High rise residential buildings are seen behind a slum in Mumbai, India, on 20 July 2010Danish Siddiqui/Reuters
World Population Day 2015
The fairground lights up during sunset on the opening day of Oktoberfest in Munich in September 2014. An estimated six million people attend the 16-day festivalJoerg Koch/Getty Images
World Population Day 2015
Hindu pilgrims make their way over pontoon bridges near Sangam during the Maha Kumbh Mela in Allahabad, IndiaDaniel Berehulak/Getty Images