The criminal case against the Kneecap rapper Liam Óg Ó hAnnaidh should be thrown out, a court has heard.

Ó hAnnaidh, who performs with the band under the stage name Mo Chara, will have to wait until 26 September, when the chief magistrate, Paul Goldspring, will rule on whether he has the jurisdiction to try the case.

The Belfast musician was greeted by hundreds of supporters as he arrived at Westminster magistrates court on Wednesday for a three-hour hearing.

The Metropolitan police arrested a man outside the court on suspicion of supporting the proscribed group Palestine Action.

A force spokesperson said: “A man remains in police custody after being arrested on suspicion of supporting a proscribed terrorist group, Palestine Action, in Marylebone Road, NW1, this morning.”

Surrounded by officers, he told reporters his handmade sign made no reference to any proscribed organisations.

Prosecutors allege Ó hAnnaidh, 27, displayed a flag in support of the proscribed terror organisation Hezbollah at a gig at the O2 Forum in Kentish Town, north London, last November. Ó hAnnaidh, of Belfast, is yet to enter a plea to the charge and is on unconditional bail.

During the hearing on Wednesday, his defence team argued the case should be thrown out, citing a technical error in the way the charge against him was brought.

Brenda Campbell KC told the court the attorney general had not given permission for the case to be brought against the defendant when police informed him he was to face a terror charge on 21 May. She said consent was given the following day, but that meant the charge fell outside the six-month timeframe in which criminal charges against a defendant can be brought.

The prosecutor Michael Bisgrove said permission was not required until the defendant’s first court appearance and that permission did not need to be sought in order to bring a criminal charge.

Hundreds of Kneecap supporters waving flags and holding banners greeted Ó hAnnaidh as he arrived at court alongside his Kneecap bandmates, Naoise Ó Cairealláin and JJ Ó Dochartaigh.

Demonstrations in support of the rapper were organised outside the court building in London, as well as in Dublin. The Metropolitan police imposed conditions limiting where the demonstration outside the court could take place, saying they were needed to “prevent serious disruption”.

Fans held signs reading: “Free Mo Chara” while others waved Palestine and Irish flags before the rapper’s arrival at court.

Ó Cairealláin and Ó Dochartaigh sat at the back of the courtroom with three others as Ó hAnnaidh confirmed his name, date of birth and address to the court at the start of the hearing.