In the two-day referendum, around 6.6 million voters have been urged to choose between "Yes" and "No" to the question: "Do you approve Serbia's new constitution?"
The draft, adopted unanimously by Serbia's parliament a month ago, proclaims Kosovo an "integral" part of the country.
"I especially call on all citizens of Serbia, whatever their religion or nationality, to vote for the new constitution for Kosovo to express the unanimous will of the whole of Serbia that nobody can grab our territory and nobody can divide Serbia," Serbian Prime Minister Vojislav Kostunica said Friday.
The timing of the charter vote is seen by critics as a Serbian government ploy to avoid losing Kosovo, or at least delay the status talks and show its people it attempted to prevent Kosovo's independence ahead of elections.
Ethnic Albanians, who constitute about 90 percent of Kosovo's population, have been excluded from voting by Belgrade as they have boycotted all Serbian elections since 1990.
Kosovo has been run by the United Nations since a NATO bombing campaign in 1999 ended a brutal crackdown by forces loyal to former Serbian president Slobodan Milosevic against the Albanian majority.
Its status is being decided in UN-backed talks expected to wind up by year's end.
Belgrade is staunchly opposed to the independence of Kosovo, considered the cradle of Serbian nationhood, only offering Albanians broad autonomy.
Legal experts have said the Kosovo articles of the constitution are mostly symbolic, but the document's authors insist it represents a "compromise between two totally opposed political poles of today's Serbia".
The charter also defines Serbia as an independent state for the first time since 1918.
It comes after a decade of wars that tore apart communist Yugoslavia. Serbia's traditional partner, Montenegro, also separated from Belgrade earlier this year.
A number of opposition parties and non-governmental groups have called for a referendum boycott, citing inadequacies in justice, human rights and local autonomy issues.
The charter will be approved if backed by more than 50 percent of the electorate.
An opinion poll by the independent Center for Free Elections and Democracy showed some four million of Serbia's 6.6 million voters are expected to cast ballots at the referendum, with about 3.6 million voting "Yes".

Copyright AFP (Agence France-Presse), 2006