More than 50,000 demonstrators are expected to descend on Cologne to protest against the politics of the Alternative for Germany (AfD) as the party holds one of the most important conferences in its four-year history.
The AfD, known for its populist, anti-immigration stance, is hoping to enter parliament for the first time in Germany's general election on September 24.
About 4,000 police officers have been deployed to avert clashes on the city's streets between anti-AfD protesters and party supporters.
About 600 AfD delegates are expected to attend the conference.
The central city hotel, where the conference is being held, has made arrangements to allow staff members to stay overnight in the hotel to minimise problems with gaining entry to the venue.
Hannelore Kraft, state premier of North-Rhine Westphalia, where the conference is taking place, and Green Party leader Cem Ozdemir were due to speak at an anti-AfD rally on Saturday.
The two-day conference comes just days after AfD leader Frauke Petry ruled herself out of spearheading the party's campaign for the September election.
Her move brought to a head a long-running internal party power struggle, which pitted Petry against the party's right-wingers.
The AfD delegates attending the conference are likely to sign off on the party's election manifesto but will have to set aside months of turmoil to agree on a team to lead the campaign.
Petry has placed a controversial motion before congress calling on the party to adopt a realpolitik approach, with the aim of transforming the AfD into a mainstream party open to coalition.
Founded in 2013 on a Eurosceptic platform, the AfD has railed against Chancellor Angela Merkel's decision to let in more than one million refugees to Germany since 2015.
In May 2016, AfD backed an election manifesto that says Islam is not compatible with the German constitution and the party also supported a call to ban minarets on mosques and the full-face veil.
AfD made huge gains in a state election in September, receiving about 21 percent of votes in the northeastern Mecklenburg-Vorpommern region, beating Merkel's party to take second place.
Rising anti-refugee rhetoric has been matched by rising violence against refugees in Germany.
Germany's interior minister said in February that the country recorded more than 3,500 attacks in total against refugees, migrants and their shelters in 2016, amounting to nearly 10 acts of violence a day: a sharp rise on previous years.
But AfD's popularity has declined as the number of new refugee arrivals has dwindled, mainly due to border closures on the Balkan overland route and a European Union deal with Turkey to halt the flow.
All of Germany's mainstream parties have ruled out working with the AfD should it clear the 5 percent hurdle to representation in the election.
Opinion polls show the AfD at between 7 and 11 percent, a steep drop from the 15 percent support it drew only late last year.
|Cologne is widely regarded in Germany as a largely liberal city [Ralph Orlowski/Reuters]|
Al Jazeera's Paul Brennan, reporting from Cologne, said it was questionable whether the AfD would make it into parliament in the upcoming elections.
"In recent months they've had internal divisions, which have hamstrung them a little bit," he said.
"And the other thing is that as they have grown, opposition against them has also grown."
Our correspondent said that while the party as a whole has generally tried to avoid controversy, many individual AfD members have publicly taken positions that many find offensive.
"The feeling among many critics is that individuals within the party are essentially neo-Nazis, and clearly in Germany that's a very sensitive issue," he said.
Some scuffles occurred on Saturday morning between protesters and police - most of whom are in full riot gear - as tensions were high in the city, which is regarded as widely liberal.
"People are very angry that the AfD are here. They are determined to try to stop delegates reaching the hotel. It's going to be a very tense day," Al Jazeera's Brennan said.
Source: Al Jazeera and news agencies