They were led by Jens Stoltenberg, the head of Nato, who said Britain's role at the heart of Europe is crucial in combating terrorism and illegal mass migration.
He also said a fragmented Europe would exacerbate instability in the region.
"I don't have a vote. It's up to the people of Britain to decide. What I can do is tell you what matters for Nato, and a strong UK in a strong Europe is good for the UK and it's good for Nato, because we are faced with unprecedented security challenges, with terrorism, with instability and an unpredictable security environment, and a fragmented Europe will add to instability and unpredictability."
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Stoltenberg's intervention comes with final polls showing a statistical dead heat, with around 10 per cent of voters undecided, and a lean towards Remain in the final polls. A YouGov gives Remain a two per cent lead while FTI gives Remain a three per cent lead.
Elsewhere, ComRes gives Remain an eight per cent lead and Opinium puts Leave in front by one per cent.
On Wednesday, European Commission President Jean-Claude Juncker warned that the UK will not be able to return to the negotiating table if it votes to leave the EU in Thursday's In/Out referendum.
He was among a number of European statesmen to weigh in on the eve of the referendum.
France's President, François Hollande, said the EU's future was at stake on Thursday, adding, "There's a very serious risk for the UK not to be able to access the common market and the European economic area any more.
France would "draw all the conclusions" of a vote to leave, he said, adding, "This would be irreversible."
Belgian Prime Minister Charles Michel said that whatever the outcome, he would seek a special meeting of EU leaders to assess the results.
Michel said he wanted an informal meeting because "there is this clear signal all over Europe, not only in Britain" of discontent. "We feel more and more hesitation about the European project," he told the VRT network.
Yet more support for the Remain camp came from Jonathan Hill, Britain's European Commissioner, who, writing in the Guardian newspaper on Wednesday, said, "If cool heads prevail and the British people choose to stay in the EU it would mean that they have sent a message that Europe is where they expect their leaders to do a lot of their economic and diplomatic heavy lifting."
EU leaders will converge on Brussels next week for a summit where the result is expected to be top of the agenda.
A record number of Britons in the UK, Gibraltar and overseas territories are eligible to take part in the referendum and 46,499,537 people will head to the ballot boxes, according to early figures by the British electoral commission.
There is no minimum turnout needed for the vote to be valid and binding.
Polls opened at 7am on Thursday and will close at 10pm local time, when counting gets under way at the UK's 382 local voting centres.