After attacks, arm-in-arm world leaders join mass Paris march

* France on top security alert * World leaders march with French citizens * Attacks raise concern over freedom of speech, religion * Jewish leader says sites promised army protection * White House sets Feb. 18 security summit (Adds organiser on turnout) 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 an estimated million-plus French citizens through Paris in an unprecedented march under