Serbia's parliament on Monday proclaimed the Balkan republic a sovereign state, following tiny Montenegro's decision to split from a joint union and dissolve what was left of Yugoslavia.

The 126 lawmakers unanimously acknowledged that their state is the heir to the union of Serbia-Montenegro — the last shred of what was once a six-member Yugoslav federation. Serbia's parliament has 250 deputies, but the opposition boycotted the vote, walking out just before the balloting.

The walkout briefly left the ruling coalition without the necessary majority, and triggered a break in the session.

CountryWatch: Serbia-Montenegro

The session later resumed, but the slim majority of votes in favor of the government-backed declaration — even though it was just a formality — signaled the weakness of Prime Minister Vojislav Kostunica's Cabinet.

"Serbia, as the legal heir to the state union, must formally take over, or inherit, what it has created," said parliament speaker Predrag Markovic.

The assembly instructed all state institutions to complete the process for Serbia's statehood within the next 45 days, including assuming the duties and responsibilities previously in the hands of the federal administration. Minutes after the vote, the Serbia-Montenegro flags were removed from the parliament building.

Neither Kostunica nor Serbian President Boris Tadic attended the session, in an apparent bid to downplay the fact that Serbia became an independent state by default when Montenegro declared independence following its May 21 referendum.

Still, some deputies praised Serbia re-establishing its statehood after 88 years in the Balkan union.

"We will restore Serbia's glory," said Miloljub Albijanic, from the ruling G17 Plus party. "Long live independent Serbia."

But nationalist leader Tomislav Nikolic felt less proud.

"This is a sad day in the history of Serbia," he said. "Something is happening in Serbia against Serbia's will."

Montenegro's declaration of independence late Saturday set in motion the process of dividing the joint state's armed forces, diplomatic missions, common assets and responsibilities.

Serbia and Montenegro were the only two former Yugoslav republics that stayed together after the violent disintegration of the Balkan federation in the 1990s.

Although the two nations share a common language and culture, as well as close historic ties, their relations cooled over recent years, with Montenegro edging toward independence.

In May, Montenegro's voters chose by a slim margin to split from the 88-year-old union with Serbia.

In the early 1990s, Slovenia, Croatia and Bosnia also split from the Serb-led federation, triggering a series of bloody wars. Serbia's southern, ethnic Albanian-dominated province of Kosovo also hopes to gain independence at ongoing U.N.-brokered talks.

"Everybody has left us," 33-year-old Serb engineer Dusan Petrovic said Monday. "It's time we rid ourselves of all illusions."

Serbia, as the union's legal successor, inherits membership in the United Nations and other international organizations.

Meanwhile, Montenegro's President Filip Vujanovic sent a letter to U.N. Secretary General Kofi Annan asking that his newly proclaimed state be admitted as a new member in the world body.

On Sunday, the Adriatic republic asked its neighbors, European Union states and permanent members of the U.N. Security Council to officially recognize it and establish diplomatic relations with its government.