A bitter dispute over a flood of North African immigrants is widening divisions among some of Europe's most powerful nations and adding to strains on the long-held dream of a united Europe.
Since January an estimated 26,000 Tunisians have fled unrest in their country for the shores of Italy, where officials say the burden of caring for these immigrants should be shared by the 27-nation European Union.
The Italians have taken the unusual step of issuing many of the Tunisians temporary residence permits and say that those papers allow the immigrants to go anywhere in a 25-nation zone that permits legal European residents to cross borders without a visa.
The Italian stance has infuriated Germany and France, the former colonial power where many of the Tunisians want to reunite with relatives, friends and co-workers.
Neither side is backing down, tensions are rising, and on Sunday French police stopped a train carrying Tunisian immigrants from Italy at the border. It was an unprecedented affront to Europe's cherished vision of visa-free travel in a united continent where in many places there is nothing to indicate a national border beyond a roadside welcome sign.
There was a large French border police presence on the Italian frontier Monday and officers were checking the passports of all Tunisians passing through, and their ability to support themselves.
In Germany, separated from Italy by Austria and Switzerland, Interior Ministry spokesman Jens Teschke said there would be "more intensive observation" of people entering the country, though he would not give specifics. He said there were still no formal controls on borders that have been visa-free.
The state interior ministry in Bavaria, which contains all of Germany's border with Austria, said that a system of spot checks near the borders that has been in place since the visa-free zone started has been stepped up somewhat, leading to more checks on roads, train stations within some 20 miles of the border.
"It's a bit easy for Italy to be generous with other people's territory," said Christian Estrosi, the mayor of the French city of Nice, near the Italian border, and a prominent member of President Nicolas Sarkozy's conservative UMP party. "What are the consequences of this? Italy, in the name of the EU, has made an incredible offer of hope" to North African immigrants. "This is not acceptable."
A host of other disputes are simultaneously threatening the grand project of European unity: On the same day the French stopped the train at the Italian border of Ventimiglia, a party that opposes the EU and financial bailouts for struggling countries made significant electoral gains in Finnish elections.
The True Finns may well play a role in a new Finnish government, jeopardizing the effort to stabilize the common euro currency by bailing out Portugal and other nations _ perhaps the single most visible symbol of European unity.
In any event, political wrangling in Portugal may endanger its ability to negotiate the bailout that it says it needs. And in Greece, calls are growing louder for the country to ignore the financial strictures imposed on it by the EU and default on its debts instead.
While no single issue will sever the strong, deep ties between the nations of the EU, many observers see significant long-term damage to the idea of uniting vastly dissimilar cultures and economies in a single confederation.
France said it was justified in halting the train from Italy because there were pro-immigration activists on the train who threatened public order. In any event, French officials said, it would honor the Italian residence permits only if the immigrants could show they had enough money to support themselves.
The town of Ventimiglia said Sunday that it would find temporary shelter for the Tunisians who were aboard the train. It was not immediately clear Monday if all were still in the town, returning to other places in Italy or trying to get to France by other means.
Italian Interior Minister Roberto Maroni said the immigrants should be allowed to travel throughout the Schengen area.
"We have given the migrants travel documents, and we gave everything that is needed," Maroni said in an interview on Italy's Sky TG24 TV.
But Michele Cercone, a European Union spokesman, said the French appeared to be within their rights under the Schengen agreement to refuse entry to the immigrants. A temporary residence permit, Cercone said, is neither an EU passport nor and EU visa and does not grant people the right to move freely in the borderless Schengen area _ the group of 25 European countries to which both Italy and France belong.
Cercone also said the Schengen agreement allows police checks along borders, so long as they are not systematic and do not amount to border controls. And people can be prevented from crossing a border for reasons of public security, he said.
Also driving the immigration dispute is a clash between two political leaders, both with strong anti-immigrant credentials, and both in political trouble. Italian Premier Silvo Berlusconi is on trial for several alleged offenses, including, prosecutors say, paying an underage girl for sex. He has announced he might not run for re-election in 2013 _ often a sign that a politician fears being forced to leave even before the end of his term.
And Sarkozy is riding low in the polls _ behind Marine Le Pen, leader of the anti-immigrant National Front.
"A huge comedy is being played between France and Italy regarding the Tunisian migrants," said Pierre Henry, general director of France Terre d'Asile, an immigrants rights group. "Each time there is a migrant crisis in the EU, everyone sends to one another the responsibility of solving the crisis."
Angela Charlton in Paris, Alessandra Rizzo in Rome and Geir Moulson in Berlin contributed to this report.