A no-fly zone is being enforced for four days over the German city of Cologne in a huge security crackdown for an anti-immigration party’s conference.

Thousands of police officers armed with tear gas and water cannons are being deployed for Alternative für Deutschland’s (AfD) meeting, where up to 50,000 protesters are expected to greet delegates.

As well as the risk of violent clashes between party supporters, demonstrators and police, a right-wing group claiming responsibility for the Dortmund bus attack threatened the gathering.

In one of three claims for the bombing targeting the Borussia Dortmund football team, an anonymous email sent to a German newspaper said the attack was the “last warning” before “coloured blood will flow” on 22 April.

The message, which included references to Adolf Hitler and railed against multiculturalism, was believed to be a threat against left-wing demonstrations on Saturday against the AfD’s perceived racism, xenophobia and neo-Nazi links.

Supporters of the anti-Islam Pegida group and anti-fascist protesters have repeatedly clashed, while there have been more than a thousand attacks on refugee accommodation as hate crimes rise.

A government report warned of rising political violence from both the left and right wing since the start of the refugee crisis.

Police use a water cannon during a protest march by supporters of anti-immigration right-wing movement Pegida in Cologne on 9 January 2016 (Reuters)

The German interior ministry said 39,000 recorded incidents in 2015 represented a “new high” since political crimes started being recorded separately in 2001, which it attributed mainly to a 44 per cent increase in violent crime by right-wing extremists.

But the number of violent crimes committed by the left wing were even higher, rising 35 per cent to 2,246 incidents, largely directed against the police.

Officers are implementing heavy security measures over the weekend, bringing in a four-day no-fly zone over central Cologne on Thursday.

North Rhine-Westphalia Police said the ban included aircraft including commercial flights, drones and models, and would be enforced by air traffic control.

Exceptions will be made for German military and police aircraft, as well as the emergency services in the city, where tensions have run high at protests since mass sexual assaults on New Year’s Eve 2015.

The ban will run throughout the AfD convention, which will take place on Saturday and Sunday as the party pushes to turn around sliding polls ahead of September’s federal elections.

Germany reacts to Cologne New Year's Eve attacks Show all 13 1 /13 Germany reacts to Cologne New Year's Eve attacks Germany reacts to Cologne New Year's Eve attacks Women protest against sexism outside Cologne Cathedral on 5 January after the assaults Oliver Berg/EPA Germany reacts to Cologne New Year's Eve attacks Women protest against sexism in Cologne following the rash of sex attacks on New Year's Eve Wolfgang Rattay/Reuters Germany reacts to Cologne New Year's Eve attacks Police initially failed to mention the assaults in report the following morning EPA Germany reacts to Cologne New Year's Eve attacks Police officers patrol in front of the main station of Cologne, Germany AP Germany reacts to Cologne New Year's Eve attacks German far-right supporters demonstrate at Cologne`s train station (Reuters) Reuters Germany reacts to Cologne New Year's Eve attacks Supporters of anti-immigration right-wing movement Pegida in Cologne, Germany, January 9, 2016. Reuters Germany reacts to Cologne New Year's Eve attacks Police used pepper spray to control supporters of Pegida, Hogesa (Hooligans against Salafists) and other right-wing populist groups as they protested against the New Year's Eve sex attacks on 9 January, 2016 in Cologne, Germany Reuters Germany reacts to Cologne New Year's Eve attacks Police use a water cannon during a protest march by supporters of anti-immigration right-wing movement Pegida in Cologne, Germany, January 9, 2016 Reuters Germany reacts to Cologne New Year's Eve attacks Police use pepper spray against supporters of anti-immigration right-wing movement Pegida, in Cologne, Germany, January 9, 2016. Reuters Germany reacts to Cologne New Year's Eve attacks Artist Mira Moiré protests naked in Cologne against the mass sex attacks on New Year's Eve AP Germany reacts to Cologne New Year's Eve attacks A demonstrator holds a sign in German that reads 'No violence against women' during a demonstration in the wake of the sexual assaults on New Year's Eve, outside the cathedeal in Cologne, Germany, 09 January 2016. EPA Germany reacts to Cologne New Year's Eve attacks Counter demonstrators hold up a sign reading "Against sexism, against racism" as they protest against a demonstration of the islamophobic movement PEGIDA at the train station in Cologne, Germany, on January 9, 2016. AFP/Getty Images Germany reacts to Cologne New Year's Eve attacks Demonstration by a women’s group on Saturday (AP) AP

Opinion polls show the AfD winning enough votes to enter the Bundestag for the first time, after it enjoyed unprecedented success in local elections on a heavily anti-immigration and Eurosceptic campaign.

A dramatic decline in the number of refugees arriving in Germany has coincided with waning support, while the party has also been hit by infighting and controversies over members’ attitude to the Nazis.

A fresh blow was dealt on Wednesday, when the AfD’s co-leader Frauke Petry said she would not lead its election campaign despite being considered a serious threat to Angela Merkel’s party.

Ms Petry, the international face of the party, had caused controversy among members by tabling a motion for its conference saying the AfD should be open to join future coalitions.

Shunned by the German mainstream left-wing and conservative blocks, other AfD members want to be a “fundamental” opposition party.

Ms Petry, who is pregnant with her fifth child, denied suggestions that she had made the proposal with a view to becoming the party’s top candidate.

Frauke Petry has a reputation for appearing entirely reasonable while supporting the xenophobic far right (AFP/Getty)

“In order to put an end to all speculation in this regard, I am using the opportunity of this video message to clearly state that I am neither available for a lone lead candidacy nor for participation in a top team,” the 41-year-old said in a video posted on Facebook.

Ms Petry said she had decided not to run as it was important for the AfD to discuss issues such as her proposal on the party’s future strategy – which the majority of regional AfD branches oppose – without being hindered by personnel questions.

Analysts expect her party to struggle to find a replacement matching her public profile within the crucial coming months, and say it could be damaged by the appearance of a far-right candidate.

Ms Petry’s camp wants to expel a senior party member, Björn Höcke, for calling Berlin’s Holocaust memorial a “monument of shame” and saying Germany should take a more “positive” attitude to its Nazi past.

She managed to secure a two-thirds majority on the party executive board in favour of expelling him but the AfD’s far-right wing supports Mr Höcke and a party arbitration board must now decide his fate.

Originally an anti-EU party, the AfD has been bolstered by attacking Ms Merkel’s decision to open Germany’s borders to Syrian refugees in 2015, which saw more than a million asylum seekers arrive in the country.