German Chancellor Angela Merkel's Christian Democrats were beaten into third place by the anti-immigrant and anti-Islam Alternative for Germany (AfD) party in a north-eastern state election on Sunday, TV exit polls showed.

In a stinging defeat for Merkel in her home district that could weaken her chances of a fourth term in next year's federal elections, the upstart AfD took 21.9 percent of the vote behind the center-left Social Democrats (SPD) in their first election in Mecklenburg-Vorpommern by campaigning hard against the chancellor's policies on refugees, according to a projection by ARD TV at 1815 GMT.

"This isn't pretty for us," said Michael Grosse-Broemer, one of Merkel's top deputies in parliament in Berlin in a ZDF TV interview. "Those who voted for the AfD were sending a message of protest."

Merkel's approval rating has plunged to a five-year low of 45 percent, down from 67 percent a year ago, due to spreading disenchantment with her open-door policies on refugees.

According to a Der Spiegel magazine report, Merkel wanted to announce her intention of running for a fourth term this year but put that on hold due to resistance from her Bavarian sister party, the Christian Social Union. The arch-conservative CSU has demanded that Merkel put limits on the numbers of refugees.

"This was a dark day for Merkel," Thomas Jaeger, a political scientist at Cologne University, told Reuters. "Everyone knows that she lost this election. Her district in parliament is there, she campaigned there, and refugees are her issue."

The election took place exactly a year after Merkel's decision to open Germany's borders to hundreds of thousands of refugees and the discontent in the state was palpable.

"This is a slap in the face for Merkel -- not only in Berlin but also in her home state," said Frauke Petry, co-leader of the AfD. "The voters made a clear statement against Merkel's disastrous immigration policies. This put her in her place."

The AfD's win was cheered by the leader of France's far-right National Front party, Marine Le Pen, who posted on Twitter: "What was impossible yesterday has become possible: the patriots of AfD sweep up the party of Ms Merkel. All my congratulations!"

The SPD, which has ruled the rural state on the Baltic coast with the CDU as junior coalition partners since 2006, won 30.2 percent of the vote, down from 35.6 percent in the last election in 2011. The CDU won 19 percent, down from 23 percent in 2011, and its worst result ever in the state, ARD TV said.

The Left Party won 12.7 percent, down from 18.4 percent five years ago, while the Greens won 4.9 percent, down from 8.7 percent, and fell out of the assembly. The far-right NPD was also knocked out of the state legislature, falling below the 5 percent threshold for the first time since 2006.

Despite losing support, the SPD (26 seats) and the CDU (16) won enough seats to be able to continue their coalition in Mecklenburg-Vorpommern, with the AfD as the second-largest bloc in the 71-seat state assembly with 18 seats. The SPD, which could also form a coalition with the Left Party, said it was leaving its options open.

Voters already punished Merkel in three state elections in March, voting in droves for the AfD and rejecting Merkel's Christian Democrats.

Founded in 2013, the AfD now has won seats in nine of the 16 state assemblies across the country. However, it has no chance of governing in Mecklenburg-Vorpommern since the other parties have said they would not form a coalition with the party.

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The AfD is also making gains nationwide, a new poll showed on Sunday. If the national election were held next week, the AfD would win 12 percent of the vote, making it the third-largest party in Germany, according to a poll conducted by the Emnid institute for the Bild newspaper and published on Sunday.

Merkel had made a last-minute campaign appearance on Saturday in Mecklenburg-Vorpommern, warning against the politics of "angst" offered by AfD with its virulent anti-refugee stance.