The BBC's David Eades reports from Denmark

They are voting on whether to ratify the European Union's Amsterdam Treaty, which would allow the EU to take in new members from eastern Europe.

The treaty also calls for the introduction of common European policies on immigration and asylum.

A "No" vote would put the future of Europe as outlined in last year's Amsterdam Treaty in doubt because it would slow up ambitious plans to expand the union.

Uncertain outcome

On the eve of the vote, Denmark's Prime Minister Poul Nyrup Rasmussen said the race was too close to call.

"Think about Denmark's future. We do not want to be isolated," Mr Rasmussen told a woman as he handed out roses in Copenhagen in one of his last campaign appearances.

However, he said the referendum result should be respected because it would reflect the electorate's decision.

Lutheran clergyman campaigning for 'No' vote

"No" campaigners seemed encouraged by opinion polls which showed the gap narrowing between them and supporters of the treaty.

However, opinion polls before Denmark's shock "No" vote in 1992's referendum on the Maastricht treaty proved wrong when they showed the "Yes" campaign leading.

Analysts said up to two thirds of the four-million-strong Danish electorate were wavering between their wish to be part of the EU and their intense dislike of union interference in Danish affairs.

They said treaty supporters had remained clearly in the lead in opinion polls but the gap narrowed in some surveys leading up to the vote.

The Amsterdam Treaty, which was agreed a year ago, would give the EU authority over visa and asylum policies in the member countries, and paves the way for Eastern European countries to join.

All members need to ratify

The treaty must be ratified by all the 15 members before it can take effect.

Most EU countries have left it to their parliaments to ratify the treaty.

However, the Danes are obliged to hold a referendum because it involves sovereignty being ceded to European institutions.

Police in Copenhagen equipped with riot control gear are prepared for violent protests that could greet a "Yes" result - as happened during the 1993 referendum when Denmark finally endorsed the Maastricht Treaty on European economic and monetary union.

After the vote five years ago, leftist anti-EU protesters fought for five hours with police who opened fire, wounding 11 people. Thirty police were also hurt.