Officers clashed with demonstrators who staged sit-ins to obstruct roads to the Alternative for Germany (AFD) conference in Hannover, delaying the start of the meeting by around an hour.
Several officers were injured in the unrest, including one who was struck on the hand by a bottle, while a demonstrator who chained himself to a barricade suffered a broken leg and was taken to hospital.
Later, more than 5,000 pro-refugee demonstrators marched through the city centre to oppose AfD, an anti-immigration and anti-Islam party founded in 2013.
Using slogans such as "Bikinis Not Burqas", "Stop Islamisation" and "Merkel must go", AfD won its first seats and achieved a shock third-place result in September's election.
But less than 24 hours after the result, co-leader Frauke Petry resigned saying there was a "disagreement over content".
At the Hannover conference this weekend, party members will vote on whether to replace or re-elect current leader Joerg Meuthen.
He faces a challenge from the party's Berlin chief and former army colonel Georg Pazderski, while there were also rumours AfD parliamentary group chief Alexander Gauland could mount a leadership challenge.
As the conference began, Mr Meuthen told delegates that AfD was Germany's "only party for patriotic policies" and accused Chancellor Angela Merkel of "fundamental political failure" during her 12 years in office.
Predicting that the party would hold seats in all 16 states by the end of 2018, he added a collapse of coalition talks involving Mrs Merkel had boosted support for AfD.