The chairman of a Dutch Jewish group said he had been assaulted by a prominent ally of Jeremy Corbyn, the leader of Britain’s Labour party, during that politician’s visit to the Netherlands.

Hidde J. van Koningsveld, chairman of the CiJo student group, complained to police about the alleged altercation with writer Paul Mason on Tuesday, at an event hosted by the Dutch Labor party, PvdA. Mason denied the allegations.

Van Koningsveld told police on July 10 that five days earlier, three people he did not know shouted at him and shoved him around at the Paard nightclub in the Hague. The three men approached him and two of his friends because they unfurled during Corbyn’s speech there a banner reading: Labour — for the many, not the Jew to protest the British party’s anti-Semitism problem. Many British Jews blame Corbyn for it.

One of the three men “deliberately, clearly, and forcefully hit me on the back of my neck,” van Koningsveld told police. “It hurt,” he added, but he did not seek medical attention.

On July 12, van Koningsveld wrote on Twitter that he had learned that the main culprit was Paul Mason, a former journalist for the BBC and Channel 4 and Corbyn supporter, who also spoke at the event in the Hague. A CiJo spokesperson wrote on Twitter that Mason’s name has “been forwarded” to Dutch police.

Mason, an author and frequent contributor to major newspapers in the United Kingdom who has more than 500,000 followers on Twitter, denied the accusation outright, publishing on Thursday a declaration titled “Statement on Hague meeting allegations: false, defamatory, evidence free and concocted.” In it, he wrote: “Claims that I punched somebody are concocted, ludicrous and without evidence.”

According to Mason, he did interact with the three protestors to tell them that he objected to the sign. He had thought upon seeing van Koningsveld’s sign that “a small group had hung an anti-Semitic banner, presumably to embarrass Labour,” Mason wrote in the statement.

In his statement, Mason also wrote that although one of the three protesters “half-heartedly elbowed me in the chest, I at no point retaliated.” In the same statement, he also wrote that throughout his interaction with van Koningsveld, “the situation was tense but peaceful at all times.”

According to an account of the altercation by David Garcia a supporter of Corbyn who apparently was also present at the event, “Paul Mason who was also present was understandably furious and tried to pull it down.” Garcia added a photograph of the banner to his tweet about Mason.

Paul van der Bas, another pro-Israel activist who was with van Koningsveld during the altercation with Mason, said that he and at least three others saw Mason assaulting van Koningsveld.

“You got physical as soon as you came up to us. Shouted at us and grabbed and shoved us. You kept standing very close even as we asked you to keep some distance. Then you punched Van Koningsveld in the neck repeatedly,” van der Bas wrote on Twitter to Mason.

“Surely your fans and followers will believe you, but you and I know that you crossed a line last week. You were violent even as I asked you to keep distance multiple times. No excuse,” van der Bas also wrote on Twitter.

Van der Bas is the previous head of CiJo, the group now headed by van Koningsveld. CiJo is a sister association of the Center for Information and Documentation on Israel, or CIDI. CiJo focuses on students, youths and young adults. CIDI is the main watchdog on anti-Semitism in the Netherlands. It is also an Israel-advocacy lobby group. Its current director, Hannah Luden, has been a longtime member of Dutch Labor.

In May, Jonathan Arkush, who was then still president of the Board of Deputies of British Jews, said that Corbyn “has views which are anti-Semitic.” The Board’s current president, Marie van der Zyl, has accused Corbyn of trying to “whitewash” his party’s anti-Semitism problem. Corbyn defended an anti-Semitic mural in 2013 and called Hamas and Hezbollah his “friends” while hosting representatives from those groups as his guests in parliament in 2009. He has also vowed to kick out from the party members who are caught making racist statements about Jews and others.

Mason referenced his Jewish ancestry in his statement. “As the descendant of Polish Jews I have, though not Jewish, experienced anti-Semitism at school,” he wrote. “My commitment to fighting fascism and anti-Semitism, to remembering the Holocaust and Jewish resistance to it, is well established.” In a 2015 op-ed for the Guardian, Mason wrote: “I am the grandson of a Lithuanian Jew on one side, and some miners and weavers” from the United Kingdom.