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Hungarian police block the rail tracks on the border with Serbia as construction workers finish the fence outside Roszke, Hungary, on Monday.Credit Sergey Ponomarev for The New York Times

BUDAPEST — For three years, Hussam Haffi had been living in a Lebanese camp for those displaced by the war in Syria, eager to move his wife and four children to Europe, but wary of the journey’s many dangers. Finally, with Hungary threatening to crack down on migrants beginning Tuesday, Mr. Haffi decided it was time to act.

“We heard yesterday that Hungary was closing the border, so we hurried up,” said Mr. Haffi, a 44-year-old Syrian banker.

But not until he had managed to slip across the thickly forested border from Serbia on Monday did he hear that Germany and several other nations had imposed new border restrictions.

“I had thought of going to Germany, or maybe Sweden,” Mr. Haffi said, shrugging with resignation as his family was being loaded onto a police bus at the border. “Now, I don’t know. Maybe Austria is a good country?”

Plans were being upended Monday all along the dangerous trail that has funneled tens of thousands of migrants this year from the Middle East, Afghanistan, Pakistan and Africa across the West Balkans into the heart of Europe. The news that Germany — and later Austria, Slovakia and the Netherlands — had imposed border controls passed like an electric charge through the human chain of refugees.

Hungary Enacts Tougher Border Controls

“If we can’t get to Germany, we are thinking of going to Belgium,” said Dana Jawad, a former sociology student from Damascus. Whatever happened, she would not return to Syria. “We had to leave,” she said. “The war destroyed everything.”

Just as they had when previous obstacles blocked their path, migrants interviewed along the route Monday were vowing not to give up. Some said they would plow ahead and hope to slip into Germany somehow, others talked of finding new routes that bypass Hungary by either moving west into Croatia and Slovenia or across the land bridge from Turkey into Bulgaria.

In Hungary, where thousands had passed in recent days along a rail line that once carried the Orient Express, the police late Friday afternoon sealed off that gap in the border fence and told newly arriving migrants to cross only at legal border stations.

Under new regulations passed by the Hungarian Parliament that go into effect Tuesday, those convicted of crossing the border illegally or damaging the border fence could be sent to prison for years. A new system of “transit centers” was also to be built at the border to hold migrants there while their cases are rapidly investigated, with those rejected to be sent back to Serbia.

Speaking to police officers on Monday, Prime Minister Viktor Orban told them to treat the migrants with humanity but be “uncompromising.”

At the same time, though, Hungarian officials seemed eager to get rid of many of the migrants already in the country. Hungary mounted at least one special, 12-carriage train to carry more than 1,000 people directly from the Serbian border to the Austrian frontier in a few hours.

By late evening, more than 1,000 migrants were waiting beside a line of buses at the Roszke train station. A second train was supposed to arrive to carry them all across Hungary to the Austrian border. Officials said that seven trains had carried migrants from Roszke to Hegyeshalom, on the Austrian border, since early Monday morning.

By Monday afternoon, the once-teeming migrant encampment at Budapest’s Keleti station had dwindled to a handful, with volunteers outnumbering migrants and people moving swiftly through the station and onto trains headed west. Free train tickets were handed out.

“They don’t stay very long,” said John Henderson, an American living in Budapest who has been volunteering to help migrants. “They get on a train as fast as possible.”

Rajub Karimi, 21, from Damascus, was handed a train ticket and a bowl of hot food. He did not linger. “I’m here to go to Germany,” he said and headed down the platform.

In Macedonia and Serbia, similar efforts were underway to move the migrants through their countries as rapidly as possible. Macedonia had changed its laws to allow the migrants to pass through the country unimpeded, and even mounted seven trains to carry an estimated 4,000 migrants to the Serbian border.

In Serbia, the authorities brought busloads of migrants to the railroad crossing at its northern border until just minutes before the Hungarians closed that door, parking a freight car festooned with razor wire on the tracks to plug a gap in the border fence.

In Edirne, in northwest Turkey near the start of the migrant route, migrants were weighing the news out of Hungary and Germany and debating whether to rush ahead or to wait.

Leila Hess, 38, a Syrian who had been living in Istanbul for three years with her five children, waiting for her chance to cross into Europe, said she was shocked to hear about the new German border restrictions.

Graphic Seeking a Fair Distribution of Migrants in Europe German and European Union leaders have called for European countries to share the burden of absorbing the hundreds of thousands of migrants who have poured into the continent this summer.