Montenegro began voting in parliamentary elections Sunday with opposition groups hoping to end the quarter-century rule of pro-Western premier Milo Djukanovic, who warns that his rivals would derail imminent NATO accession.

NATO on Wednesday invited Montenegro to become the 29th member of the U.S.-led military alliance, defying Russia's warnings it would have to respond to what it branded a threat to its security.

Russia quickly said it would be forced to react to NATO's expansion eastward, with the invitation to the small Balkan country adding to bad blood between Moscow and the West over a host of issues including Ukraine.

Around 3,000 people rallied in the Montenegrin capital of Podgorica on Sunday to back an opposition call for the left-wing government of Prime Minister Milo Djukanovic to resign.

The main opposition party in the tiny Adriatic nation is demanding the installation of a transitional government charged with organizing "genuine elections" which the opposition alleges has not been the case to date.

Croatia on Saturday announced an unprecedented spike in the arrival of migrants on a long, dangerous journey towards western Europe as the continent's worst post-World War II refugee crisis showed no sign of abating.

With many of the people fleeing war and misery flocking towards Germany, a new poll showed Chancellor Angela Merkel's popularity at home has been hit by her policy of openness.

Russia on Thursday broadened a food embargo imposed in retaliation for Western sanctions over Ukraine to include Iceland, a significant fish importer, as well as Montenegro, Albania and Liechtenstein.

Prime Minister Dmitry Medvedev said Ukraine will be added to the embargo from January 1 next year when a landmark EU-Ukraine trade deal enters force, unless Kiev makes a deal with Moscow.

An explosion has shattered the offices of an opposition newspaper in Montenegro, which is highly critical of the country's authorities and has been the target of other attacks, officials said Friday.

No one was injured in the blast caused by an explosive device which took place shortly before midnight (2300 GMT) on Thursday, police said in a statement, adding that an investigation was under way.

Police in Montenegro said they arrested 22 people Wednesday on suspicion of smuggling hundreds of migrants into Schengen, Europe's visa-free travel area.

The networks busted with arrests in the capital Podgorica and two northern towns of Danilovgrad and Rozaje consisted of locals and four people from Sudan, Pakistan and Morocco, spokeswoman Tamara Popovic told reporters.

Sixteen people died and 31 others were injured when a Romanian bus crashed in a Montenegro gorge on Sunday in one of the most deadly road accidents in the tiny Adriatic country.

The victims, all Romanian citizens, were passengers in a bus carrying tourists, mostly elderly, for a five-day visit to the Montenegrin coastal town of Tivat, police said.

A prominent Montenegro human rights activist said Thursday she had been fined 550 euros ($710) for kissing an on-duty policeman during a protest.

In a stand-off with police during an anti-government protest in the tiny Adriatic republic last year Vanja Calovic took the policeman by surprise and kissed him in an effort to defuse the tension.

Montenegrins began voting Sunday in a presidential election amid a sluggish economy and high unemployment in the tiny Adriatric EU-candidate country.

Incumbent Filip Vujanovic, a close ally of veteran Montenegrin leader Milo Djukanovic, is tipped to win a third term. He faces former foreign minister Miodrag Lekic, who has managed to get the main opposition groups to back his candidacy.