French, foreign leaders walk arm-in-arm as millions protest Paris attacks

* At least 3.7 million attend biggest-ever demonstrations * World leaders march arm-in-arm with Hollande in Paris * Attacks raise concern over freedom of speech, religion * White House sets Feb. 18 security summit (Updates attendance numbers, adds Renzi, Netanyahu quotes) By Ingrid Melander, Sybille de La Hamaide and Julien Ponthus PARIS, Jan 11 (Reuters) - World leaders including Muslim and Jewish statesmen linked arms to lead more than a million French citizens through Paris in an unprecedente