The European Union unlocked its door to Serbia and Kosovo on Monday in reward for striking a landmark agreement to normalise ties and turn the page on the last searing trouble-spot in the Balkans.
But in the ethnically split northern Kosovo town of Kosovska Mitrovica, 10,000 angry Kosovo Serbs protested the accord, accusing Belgrade of "betraying" them and "abandoning" its former province.
In Brussels, the deal garnered praise from EU ministers and officials after the European Commission recommended the launch of formal negotiations on Serbia's entry into the EU, and talks to strike a special pact with Kosovo.
This "is a game-changer for Kosovo, for Serbia, for the whole region," said the commissioner for enlargement, Stefan Fuele.
"The rubicon has been passed," said Swedish Foreign Minister Carl Bildt, a key longtime player in EU-Balkans relations. "I think it is a very big achievement."
However officials also cautioned that seeing the normalisation of ties through would be a test for Kosovo and Serbia, which fought a 1989-1990 conflict.
"Implementation will be difficult," Bildt admitted, while British Foreign Secretary William Hague stressed that "it is now up to all involved to get on with implementing" the pact.
Serbia hopes to be given a date to kick off long-awaited EU accession talks at a summit in June after clinching the deal with Kosovo last week, the last remaining hurdle in lengthy efforts to win a berth in the bloc.
The two sides are to release a roadmap later this week on their plans to set in motion the 15-point agreement between them.
Kosovo, whose 2008 declaration of independence is recognised by all but five of the 27 EU states, has its eyes on a Stabilisation and Association Agreement with the EU -- less than membership, but a big step towards embracing the bloc.
"Kosovo has taken very significant steps towards visible and sustainable improvement in relations with Serbia," a European Commission report said.
Slovenia is currently the lone member of the former Yugoslavia to have entered the bloc, although Croatia is set to become the 28th EU state in July.
The green light from Brussels came as the governments of both Serbia and Kosovo approved the deal brokered by EU foreign policy chief Catherine Ashton aimed at settling the fate of some 40,000 ethnic Serbs in northern Kosovo who refuse to recognise Pristina's authority.
But Kosovo Serbs were bitter at being what they saw as pawns in a wider politico-diplomatic game.
"Kosovo is the heart of Serbia" and "No to Pristina, no to the Brussels accord," said slogans waved by the thousands of protesters in Kosovoska Mitrovica.
The influential Serbian Orthodox Church said the accord represented "Serbia's surrender".
The text of the accord has yet to be made public by the EU. But according to media in Kosovo, the Kosovo Serbs will have their own police and justice representatives, working under Pristina's authority, in the few areas where they make up a majority of the population.
Friday's breakthrough pact, brokered after two years of efforts and many hiccups, was initialled by Kosovo prime minister Hashim Thaci and Serbian Prime Minister Ivica Dacic.
It won immediate praise from around the world.
"What we are seeing is a step away from the past and, for both of them, a step closer to Europe," said Ashton.
US Secretary of State John Kerry welcomed it and called on both sides to "implement expeditiously and fully all dialogue agreements."
UN Secretary-General Ban Ki-moon said he wanted to "congratulate and commend" both sides for their "steadfast determination" and hoped the deal would "bring about a brighter future and lasting stability to the region."
The two premiers met in Brussels on Friday with NATO chief Anders Fogh Rasmussen, who said the alliance stood ready to help implement the accord.
NATO launched a bombing campaign against late Serbian strongman Slobodan Milosevic's forces to end the 1989-1990 conflict in Kosovo, and then set up the KFOR force, now reduced to some 5,000 troops, to ensure security.