crowded (heavily … region or city) n.: megalopolis

Deng’s plan envisions the megalopolis of 13 million inhabitants [Shanghai, China] as a booster engine, towing the country’s midriff and north into the 21st century. (James Walsh, Nothing in Wang Hon Gwen’s Life Belittled Him So Much As …, Time International, 10/5/1992, p. 18.)

Mr. el-Sisi has ordered the Parliament and government ministries to move from the city center to a $60 billion administrative capital under construction in the desert, 25 miles away.Mr. el-Sisi insists the new capital will ease the pressure on Cairo, a bulging megalopolis of 20 million people. (Declan Walsh, “A Decade On, Silence Fills Egypt’s Field of Broken Dreams,” The New York Times, 1/23/2021.)

This word, when used with the above clarifier, can also be a synonym for dense, populated or city.