Congress president Sonia Gandhi on Thursday led a solidarity march from the party headquarter here to former prime minister Manmohan Singh's house a day after he was named an accused in a coal block allocation case.Speaking to reporters during the march, Gandhi said: "We are here to show our solidarity.. we shall fight it legally.""We are here at the news of summon being served to the former prime minister Manmohan Singh.... He is known not just in our country but also in other countries for his integrity and probity," she added.A special court here on Wednesday summoned Singh, industrialist Kumar Mangalam Birla and former coal secretary P.C. Parakh in a coal block allocation case, terming it a "criminal conspiracy" with the objective to "do an illegal act".