Police in London have arrested 139 people after hundreds of demonstrators gathered outside the United States Embassy to oppose a government ban on Palestine Action and express support for the organisation.
The crowd assembled outside the embassy to challenge the UK government's decision to outlaw the group, turning out in significant numbers to publicly demonstrate solidarity.
turning out= 「大挙して集まる」「（催しやデモなどに）繰り出す」という意味の句動詞。
According to reports, the majority of the 139 arrests were made on suspicion of expressing support for a proscribed organisation. Under British law, a proscribed group is officially banned, meaning that showing public support or allegiance to it can lead directly to criminal detention.
on suspicion of= 「〜の容疑で」という意味で、警察が容疑者を逮捕・拘留する際の理由を示す報道・法的な定型表現。