* Hundreds gather at rally in central London * Protest goes ahead two days after attack at synagogue * Prime minister urges respect for those grieving * Dozens arrested after declaring support for banned group (Updates number of arrests in paragraph 8) By Vitalii Yalahuzian and Yann Tessier LONDON, Oct 4 (Reuters) – London police arrested hundreds of protesters for supporting a banned pro-Palestinian group at a demonstration on Saturday which went ahead despite requests to call it off after a deadly attack at a synagogue in Manchester. Two people were killed in the attack in the northwestern English city on Thursday and police shot dead the assailant, a British man of Syrian descent who counter-terrorism police said may have been inspired by extremist Islamist ideology. Organisers refused

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