BRUSSELS (Reuters) - Moldova's referendum on joining the European Union took place amid "unprecedented interference" by Russia and its proxies, an EU spokesperson said on Monday.

A very slim majority - 50.17% - voted yes in Sunday's referendum to inserting a clause in the constitution that would define EU membership as a goal, with fewer than 1.5% of the ballots still to be counted.

President Maia Sandu, who is also seeking re-election, wants Moldova, a small country nestled between EU member Romania and Ukraine, to join the bloc by 2030.

"This vote took place under unprecedented interference and intimidation by Russia and its proxies, aiming to destabilise the democratic processes in the Republic of Moldova," Peter Stano, the EU spokesperson said.

Russia denies the allegations of meddling. On Monday, the Kremlin branded Moldova's votes as "unfree".

The referendum - along with the presidential election, whose first round also took place on Sunday - are seen as a test of whether the former Soviet republic can escape Moscow's sphere once and for all.

Before the vote, Moldovan authorities said Ilan Shor, a fugitive tycoon who lives in Russia, had made concerted attempts to meddle in the referendum and election. The authorities said they had taken down online resources that hosted disinformation and had uncovered a programme in Russia to train Moldovans to take part in mass unrest.

"This is an ongoing effort from Russia and its proxies - not only in Moldova; indeed also against our countries - and it's a long-term fight. They don't have boundaries. We, as the European Union ... respect certain principles, including the laws, but Russia and its actors and its proxies do not respect that," Stano said.

(Reporting by Charlotte van Campenhout, Lili Bayer and Philip Blenkinsop; Writing by Makini Brice; Editing by Tassilo Hummel and Gareth Jones)