India train mumbai
Commuters hang onto an overcrowded suburbun railway train on the Central Railway (CR) line in Mumbai INDRANIL MUKHERJEE/AFP/Getty Images

There are now more than seven billion people on the planet, a figure that continues to grow every year. In 1950, the population was just 2.5 billion, which means a whopping growth rate of 180%.

A rising population brings about a number of challenges for governments and people alike, such as climate change, food supplies, infrastructure and much more. There is also a wealth of opportunities to be yielded from the growth in human resources.

Every year, to raise awareness of rapid population growth across the world, the United Nations (UN) marks World Population Day on 11 July. And for 2015, the theme is "vulnerable populations in emergencies".

"With nearly 60 million individuals having fled conflict or disaster, women and adolescent girls are particularly vulnerable," UN secretary-general Ban Ki-moon said.

"Violent extremists and armed groups are committing terrible abuses that result in trauma, unintended pregnancy and infection with HIV and other diseases."

In recognition of UN World Population Day 2015, IBTimes UK looks at the world's 10 most densely populated cities (people per square mile) with populations of over 500,000.

Dhaka, Bangladesh

Pop: 15,669,000 // Density: 112,700

Eid al-adha
Men lead cows at a cattle market the day before Eid-al-Adha in Dhaka, BangladeshReuters

Hyderabad, Pakistan

Pop: 2,920,000 // Density: 104,300

yoga
Nearly 5,000 Delhi Public School students perform yoga positions in HyderabadAFP

Mumbai, India

Pop: 17,712,000 // Density: 83,900

mumbai photos danish siddiqui reuters
Vendors sell clothes on the roadside at a second-hand clothing market in MumbaiDanish Siddiqui/Reuters

Kalyan, India

Pop: 2,841,000 // Density: 78,900

kalyan india
A man wearing a saffron coloured Hindu cap looks on during a mass protest by Indian Hindu hardliners in Kalyan, 60 km (40 miles) north of India's commercial capital, MumbaiREUTERS/Arko Datta

Vijayawada, India

Pop: 1,715,000 // Density: 78,000

Vijayawada india
An Indian flood affected villager sits with his livestock in the town of Ibrahimpatnam in Vijayawada during the October 2009 floodsNOAH SEELAM/AFP/Getty Images

Chittagong, Bangladesh

Pop: 3,176,000 // Density: 73,900

Bangladesh Protests
Activists of Jamaat-e-Islami, Bangladesh's biggest Islamist party, stand during a clash with police in ChittagongReuters

Malegaon, India

Pop: 653,000 // Density: 72,600

Malegaon
Police officials stand guard at a blast site outside a mosque in Malegaon, September 9, 2006.Reuters

Hong Kong, China

Pop: 7,246,000 // Density: 68,400

hong kong tiananmen
Pro-democracy activists hold candles while wearing T-shirts featuring a yellow umbrella, symbol of the Occupy Central movement in Hong KongBobby Yip/Reuters

Macao, China

Pop: 589,000 // Density: 65,400

Macao China
Visitors and exhibitors are seen at the Global Gaming Expo Asia in the world's biggest gambling hub of Macao on May 20, 2015PHILIPPE LOPEZ/AFP/Getty Images

Aligarh, India

Pop: 1,020,000 // Density: 63,800

Aligarh India
A farmer smokes a cigarette whilst sheltering from the rain under a chair while waiting for India's Congress party general secretary Rahul Gandhi (unseen) to give a speech at a rally held for farmers in Aligarh on July 9, 2011PRAKASH SINGH/AFP/Getty Images

Source: Demographia (PDF)