While the far-left Die Linke party easily won with about 30 percent, the Alternative for Germany came second with 23 percent, according to early exit polls, more than doubling its result in the previous election in 2014.

Merkel's Christian Democrats (CDU), which had always received the most votes since 1990, dipped massively on Sunday.

This put the anti-immigration party in second spot, narrowly ahead of the CDU, who won about 22 percent, and far ahead of her coalition partners, the once powerful Social Democrats (SPD), who scored only eight percent.

Die Linke leader Katja Kipping reacting to initial results putting her party in first place. Photo: DPA

The AfD's strong result came despite widespread criticism after an October 9th attack in the eastern city of Halle, where a suspected neo-Nazi gunman tried and failed to storm a synagogue then shot dead two people outside.

After the bloody attack, the commissioner for combating anti-Semitism, Felix Klein, like many other critics, argued that the AfD had trafficked in incendiary anti-Jewish sentiment.

The Thuringia campaign has been marked by anger, threats and recriminations, with CDU candidate Mike Mohring labelling the AfD's local leader, the nationalist hardliner Björn Höcke, a “Nazi”.

A triumphant Höcke told supporters on Sunday that the state, 30 years after the fall of the Berlin Wall, had voted for a second revolution, a “Transition 2.0”, and delivered “a clear 'no' to the ossified party landscape”.

'Personality cult'

The rise of the AfD has made it harder for the other parties to form a governing coalition, boosting the likely role of smaller players with single-digit results such as the much reduced SPD and the Greens.

The SPD plummeted to a new low of eght to 8.5 percent, compared to 12.4 in 2014. The Greens were at 5.5 percent (5.7 percent in 2014) and had to fear for their return to parliament.

The FDP came in at 5.0 to 5.5 percent, close to the five-percent hurdle needed to enter parliament.

Overall voter turnout rose significantly to around 66 percent, up from 52.7 percent in 2014.

In Thuringia, the only state ruled by Die Linke, the post-election situation is complicated further by the CDU's refusal to cooperate with the hard-left party, despite the relatively moderate stance of Ramelow, a folksy former trade union official.

AfD supporters in Erfurt, Thuringia's state capital. Photo: DPA

In the eastern states of Saxony and Brandenburg last month, the AfD also scored above 20 percent to become the second-largest force.

However, in both cases the mainstream parties kept a pact not to enter into government with the far-right party, a pledge they have also made in Thuringia.

READ ALSO: Far-right AfD second-strongest force in Brandenburg and Saxony

The election in the state of just over two million people was closely watched as another snapshot of the mood in the AfD heartland, especially given the role of Höcke, a former history teacher considered extreme even within his party.

Höcke, 47, has labelled Berlin's Holocaust memorial a “monument of shame” and called for a “180-degree shift” in Germany's culture of remembrance of the crimes against humanity committed by the Nazi regime.

Signalling political ambitions at the national level, Höcke has openly challenged the AfD's senior leadership and was accused of a “personality cult” after marching into a hall escorted by flag-waving supporters.

The CDU's Mohring recently declared that “to me, Höcke is a Nazi”.

With tensions running high on the campaign trail, police have been investigating death threats against Mohring and Greens co-leader Robert Habeck, and an arson attack on an AfD campaign truck.

READ ALSO: CDU election candidate receives second far-right 'death threat'

The AfD started out as a eurosceptic fringe party before reinventing itself as an anti-Islam, anti-refugee movement to capitalize on anger over a massive influx of asylum seekers in 2015.

Its populist message has resonated most strongly with voters in Germany's former communist east where resentment lingers over lower wages and fewer job opportunities.

Ramelow on the eve of the vote said that “the AfD claims to be the party that cares. But in reality, it is a party that knows nothing but outrage”.