The release of 13 Israelis and four Thai nationals, among them a nine-year-old Irish-Israeli girl, has prompted relief as the temporary truce in Gaza continues to hold.

It came as tens of thousands of people gathered once again in central London to demand a permanent ceasefire.

Later on Sunday, there will be a 90-minute march in the capital organised by the charity Campaign Against Antisemitism, with around 40,000-50,000 people expected to attend.

The gatherings come at a key stage in the conflict between Israel and Hamas, as the two sides exchange hostages and prisoners.

The hostages were transferred to Israel late on Saturday night, after Hamas delayed the release by several hours, accusing Israel of violating the terms of a truce deal.

Israel was to free 39 Palestinians later on Saturday as part of the arrangement, with a total of 50 hostages and 150 Palestinian prisoners to be freed during the four-day truce.

Nine-year-old Irish-Israeli girl Emily Hand was one of those released after being held captive by Hamas in Gaza since the conflict started several weeks ago.

Irish premier Leo Varadkar described it as a “day of enormous joy and relief” for Emily and her family.

The march in London was the first major demonstration since the Armistice Day event, which saw scenes of violent disorder after counter-protesters clashed with officers.

Hundreds also gathered outside the Egyptian Embassy in Mayfair for a demonstration by Hizb-ut-Tahrir, in the first protest by the group since October 21 after a video emerged showing a man chanting “jihad”.

There was a heavy police presence in the capital, with 18 arrests made over the course of the day for a range of alleged offences – including suspicion of inciting racial hatred and suspicion of supporting a proscribed organisation.

Deputy Assistant Commissioner Ade Adelekan said: “I would like to acknowledge the overwhelming majority who came into London today and exercised their right to protest lawfully.

“Regrettably, there was still a small minority who believed the law did not apply to them. Thanks to the efforts of our CCTV teams and other officers, a number of those are already in custody.

“Investigations into other offences are already under way and will continue in the coming days.”

Orlando Fraser, the chairman of the Charity Commission, also warned on Sunday that the watchdog will not allow “premises, events or online content to become forums for hate speech”.

Writing in the Sunday Telegraph, he said: “The Charity Commission is aware of a significant number of serious concerns about activities linked to the conflict in Israel and Gaza.

“These include charities representing communities across the religious divide, although these, to date, largely concern allegations of anti-Semitic or hate speech.

“Charities must not allow their premises, events or online content to become forums for hate speech against any community or unlawful extremism.

He said that the body would not “stand by and permit charities to be abused in this way”.