Macedonia's president has refused to allow a coalition of Social Democrats and ethnic Albanian parties to form a government because of its pledge to allow wider official use of the Albanian language.
The move by Macedonian President Gjorge Ivanov was criticized by the European Union.
In a snap election in December, the nationalist VMRO-DPMNE won 51 seats to the Social Democrats' 49, leaving neither able to form a government without parties representing ethnic Albanians, who make up a third of the population.
Social Democrat leader Zoran Zaev last week won support of three ethnic Albanian parties in the parliament after agreeing to support a bill to enable wider use of the Albanian language, a deal that triggered street protests in Skopje.
"I cannot give the mandate to somebody who threatens the sovereignty of Macedonia," Ivanov told reporters.
The VMRO-DPMNE had already tried but failed to form a coalition.
Macedonia, once a republic in the former Yugoslavia, wants to join the EU, but Johannes Hahn, European commissioner for enlargement, was unimpressed by Ivanov's decision, "In a democracy, one must acknowledge parliamentary majorities, even if one doesn't like them," Hahn said on Twitter.
Macedonia's relations with its Albania minority have long been difficult. It reached the brink of civil war during an ethnic Albanian insurgency in 2001, before EU and other diplomacy defused the situation.
Following December's vote, leaders of ethnic Albanian parties traveled to Tirana in Albania and agreed to a joint platform for negotiating with a coalition including demanding wider use of Albanian language.
Macedonia's nationalists, including former prime minister Nikola Gruevski, said the demand was unconstitutional.
Ethnic Albanian party Besa said Ivanov's decision was "alarming" and "a potential danger for the Albanian community in Macedonia."
In Skopje, thousands had protested Zaev's deal. The protests, which started on Monday, spread on Wednesday to all major towns where Macedonians make up the majority.