A police car was seen outside the NPD's Berlin office on Wednesday
German police are investigating whether a string of letters from the far-right NPD party to politicians from immigrant backgrounds have incited racial hatred.
Dozens of politicians of foreign descent are running as candidates in Germany's election this Sunday.
The NPD reportedly sent 30 of them letters advising them to "go home".
One recipient said the two-page letter contained a "five-point plan" for "moving foreigners gradually back to their home countries".
The letters were signed by a non-existent "commissioner for the repatriation of foreigners".
"We are investigating whether there is a suspicion of inciting racial hatred," said Martin Steltner, spokesman for the Berlin state prosecutor's office, quoted by Reuters news agency.
'This is my home'
The National Democratic Party (NPD) leader in Berlin, Joerg Haehnel, defended the letters, which were sent at the weekend.
"As part of a democracy we're entitled to say if something doesn't suit us in this country," Reuters quoted him as saying.
One recipient, Green party politician Oezcan Mutlu, said he was "shocked" to get the letter.
"This is my home town. I am German. I have only German citizenship. I have no country to go to," he told the BBC's Oana Lungescu.
Mr Mutlu, a lawmaker in the Berlin state parliament, is used to hate mail.
"Until recently I got them from individuals anonymously, but this time a party with the name on it is addressing me privately," he said.
"You don't know what they can do if they have your address. I'm not afraid at all, but I have to be careful."
Of some 50 million Germans eligible to vote in Sunday's poll, about one in 10 has an immigrant background.
Many are ethnic Germans from the former Soviet Union, while others are Turks whose parents came to Germany to help rebuild the country after the war.
The NPD, which has around 7,000 members, has been polling poorly ahead of Sunday's election. It has no seats in Germany's national parliament but has some seats in regional assemblies.
The NPD uses language eerily reminiscent of Nazi propaganda, our correspondent reports.
The letter says foreigners should be excluded from the welfare system and banned from owning land in Germany.