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Photo: Npps90, CC BY-SA 4.0 via Wikimedia Commons
The largest church in Serbia and one of the largest Orthodox churches in the world, under construction since 1935 and still not entirely finished. It's described as a temple (Hram) to distinguish it from the smaller St Sava Church (Tsrkva) adjacent. The crypt (downstairs from the main entrance) is an extensive, bright and polished area with walls covered with modern iconography, so it looks more like a metro interchange for Orthodox saints than a religious building. Services are held regularly here and in the Church, which you can also visit.
The Church of Saint Sava is a Serbian Orthodox church in Belgrade, capital of Serbia. It is the fourth largest Eastern Orthodox church in the world and one of the largest churches in the world by gross volume.
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Nikola Tesla (Никола Тесла, 1856-1943) made huge contributions to electric engineering, pioneering alternating current (enabling long-distance high-energy transfer), radio (underpinning today's mobile comms) and AC motors as used in household and office machinery. Half of this small museum is dedicated to Tesla's personal effects, while the other half contains models of his inventions. English-speaking tours start at the full hour and include demonstrations. Long lines build up at peak hours.
Crkva Sv Marka / Црква Св Марка
Large Serbian Orthodox church built 1931-40 in Serbo-Byzantine style. Within are the tombs of King Stefan Uroš IV Dušan (1308-55), and of King Aleksandar I Obrenović (1876-1903) and his Queen Draga Mašin.
Стари Двор
This was the residence of the Obrenović and Karađorđević princely dynasties from 1884 until 1922. It now houses Belgrade City Assembly, no visits except on business.
Narodna Skupština / Народна Скупштина
Grand neo-baroque legislative palace, built 1907-36.
Историјски Музеј Србије
Huge collection of materials about the Serbian nation from ancient times to the present. In the coming years it's intended to move to new premises on the site of the former railway station.
House of Flowers / Kuća cveća
This is in 3 buildings. Old Museum is a general display of the history and culture of the former Yugoslavia. 25 May Museum is an oh-so-Sixties building named for the official birthdate of Josip Broz Tito (1892-1980). It displays presents given to him during his 27-year presidency: he amassed quite a collection of youth relay batons. House of Flowers is his mausoleum.