BERLIN (Reuters) - The leader of the state of Bavaria warned German conservatives on Tuesday that they faced an "extremely threatening" situation after a "disastrous" state election on Sunday which he blamed squarely on Angela Merkel's open-door migrant policy.

Exposing deep rifts within Chancellor Merkel's conservative bloc, Horst Seehofer, the combative premier of Bavaria's Christian Social Union (CSU), said the chancellor had failed to respond to voters' worries about the migrant crisis.

"The situation for the conservatives is extremely threatening," Seehofer told the Sueddeutsche Zeitung daily, adding voters were fed up with "Berlin politics".

Merkel's conservatives suffered heavy losses in the eastern state of Mecklenburg-Vorpommern on Sunday, coming third behind the center-left Social Democrats and, more surprisingly, the anti-immigrant Alternative for Germany (AfD).

Thorbjorn Jagland, general secretary of the Council of Europe rights body, told a conference hosted by the foreign ministry that he was shocked about the anti-immigrant and anti-Muslim rhetoric used by AfD during the German election.

He said statements like "Islam has no place in Europe" were "the beginning of a very, very dangerous development" that contained echoes of the anti-Semitic sentiment of the last century and suggested some effort to push out millions of Muslims who had been living in Europe for years.

"What was (considered) extreme five years ago has become more mainstream," he said. "We have had it in the past. We should not allow it to develop again."

With a federal election just a year away, Merkel's ratings have fallen to a five-year low since opening German borders to about a million migrants last year and then championing a disputed EU-Turkey deal to solve the crisis.

Amid the post-election row, Seehofer has also canceled a trip to meet Russian President Vladimir Putin.

He said the state election disaster was a result of failing to draw the right conclusions after losses in other regional votes this year. Some CSU members have renewed calls for Merkel to put a limit on the number of migrants entering Germany.

The Social Democrats (SPD), junior partner in Merkel's right-left coalition, have seized on speculation about whether she may decide not to run in next year's election, although there are no obvious rival conservative candidates.

Deputy SPD chairman, Ralf Stegner, told Spiegel Online: "Mrs Merkel has clearly passed her zenith. The question is whether she still has her party behind her."

Merkel, chancellor for nearly 11 years, on Monday took responsibility for the state election result but stood by her migrant policy.. She wants the blessing of Seehofer's CSU before declaring she will stand for the chancellorship again.

With her party facing losses in a state election in the city of Berlin in two weeks, an INSA poll in Bild on Tuesday showed the conservative bloc unchanged on 30.5 percent at a national level, with the AfD up half a percentage point at 15 percent.

"Merkel can only hope that sanity - her sanity - works with voters. It's not certain," wrote top-selling Bild in an editorial.

(Reporting by Madeline Chambers; Editing by Catherine Evans and Hugh Lawson)