Officially, all citizens of the USSR were considered equal regardless of the race or ethnicity (called 'nationality' in the documents) from the very beginning or even slightly before. The RSFRS constitution proclaimed on 10th July 1918 stated in the article 22 that 'Russian Republic recognizes equal rights of all citizens regardless of their racial or national affiliation and proclaims that any privileges related to the latter or any oppression against national minorities or restriction of their rights would violate basic laws of the Republic'*. Similar provision can be found in the Constitution of RFSFR of 1925 (article 13) that was extended by the inclusion of freedom of using native languages in official and private settings). Constitution of RSFSR of 1937 maintained that 'equality of all citizens regardless of race and nationality in relation to all economic, official, cultural and socially-political activities is considered an inalienable right' while introducing explicitly outlawing any 'direct or indirect restriction of said rights and, conversely, establishment of any privileges based on the racial and national affiliation of the citizens [...] as well as any form of racial or national exclusion, hate or neglect' (article 127). Very similar provisions were also found in the Constitution of the USSR of 1936 (article 123) and 1977 (article 36). So, as long as the official law was concerned, any form of discrimination against an USSR citizen on the grounds of their race or ethnicity was outright illegal. The only exception was the period between 15th February 1947 and 26th November 1953, when the citizens of USSR were forbidden from marrying anyone holding foreign citizenship.

It should also be considered that in pre-Revolution Russia there were also no laws forbidding the interracial marriages, provided that both nupturients were Orthodox Christians (Orthodox Church was not sanctioning any marriages between Orthodox Christians and people of other faiths, including other Christian denominations). One might conclude that strong class and faith divisions in the pre-revolutionary Russian society as well as the centuries-long contact with people of other 'races' (predominantly Mongols since 13th century and indigenous Siberians since late 16th century) in the formation of the country was not conducive to the introduction of any additional divisions, e.g. one based on the racial lines.

Practice was more or less corresponding with the law. Although multi-ethnic country USSR experienced some tensions between various ethnicities, these were usually based on the cultural differences that brought various forms of stereotypization, not unlike the treatment of Italians, Poles or Irish in the USA. The most common example would be the largely non-vicious jabs against inhabitants of the northern Siberia (especially Chukchi people), usually living in their traditional communities and maintaining their old culture what usually brought the stereotype of being unsophisticated, wary of modern developments and rather naive (it is hard to draw any easily recognizable Western parallel, but the cross between the stereotypes of Poles and 'rednecks' in USA would be quite close the mark). But this was all in everyday culture, and would not be reflected in any official treatment of these minorities.

The multi-ethnic marriages were not only shunned, but they were actually considered a positive mechanism ('friendship of nations') by the communist theorists who considered them a way of modernization of the society and marginalization of the traditional way of living, especially in Central Asia in favour of the communist, internationalist ideology (what can be considered a form of ideological colonization). However, every citizen had a 'nationality' (ethnicity) mentioned in the personal documents, and in the case of people of mixed origin, after reaching 16 years of age, they had to choose between mother's and father's ethnicity to claim as their own.

The problem with pinpointing the actual number of the interracial marriages is that it was in no way traced by the Soviet authorities, who were not using the idea of 'race' in the way typical to other Western countries and focused only on the ethnicity (meaning that Russian-Ukrainian marriage was considered 'multi-ethnic' as was Latvian-Uzbek one). In the Brezhnev's era, the number of the multi-ethnic marriages was presented as the proof of the 'cultural melting pot' that USSR was meant to be as well as the corroboration of the internationalist ideas. Of course, given the limited mobility withing the country, popularity of such marriages was tightly related to the ethnic composition of the respective areas of the country, with Central and Eastern Asian republics, where distinctly Asian ethnicities were far more prevalent than in Western parts of the RSFSR. For example, in Kazakh SSR the percentage of the multi-ethnic marriages was 14.4% in 1959 and rose to 23.9% in 1989 (with the percentage of ethnic Kazakhs being roughly 40% in 1989). In Uzbekistan, the percentage of mixed-ethnicity marriage between 1970 and 1979 was rough 10.5%. It is interesting that in such cases, it was noted that it were usually 'European' men who were marrying local (i.e. Central Asian) women rather than other way around. It was also noted that the multi-ethnic marriages were far more common among the intelligentsia than in other strata of society.

*I don't have official English versions handy, so my translation may differ slightly from the accepted official wording.

Bibliography:

Ganckaya O.G., Terent'eva L.N., Межэтнические браки и их роль в этнических процессах [Interethnic marriages and their role in ethnic processes]. In: Современные этнические процессы в СССР [Contemporary ethnic processes in the USSR]. Наука, Moscow, 1977.

Lourie S.V., Дружба народов в СССР: национальный проект или пример спонтанной межэтнической самоорганизации [The Friendship of Peoples in USSR: National project or example of spontaneous interethnic self-organization] in: Общественные науки и современност [Social sciences and contemporary world]. №4 (2011).

Narynbaev A.I., Опыт социологического исследования межнационального брaкa [The experience of sociological research of interethnic marriage]. In: Altmyshbaev, A.A. (ed.), Нация и национальные отношения [Nation and National Relations]. Илим, Frunze, 1966.