Prime Minister Syed Yusuf Raza Gilani on Saturday termed Karachi as the biggest challenge and urged the nation to support the operation of the Sindh government.It is a provincial initiative which will have full backing of the federal government. The solution of Karachi lies within rule of law, he said.In his opening statement at the Council of Common Interests meeting held at the PM Secretariat here, Gilani said while the government was engaged in efforts to strengthen democracy, anti-state elements were busy in creating situations that are detrimental to economic growth and stability. We need to raise our guard against these disruptive forces so as to move forward unimpeded, he said.