When the national anthem is sung, people are required to stand up.

Answer

Standing for the national anthem is haraam. It is a form of idolatry. Even if there is no kufr in the song, standing in respect for something which is not valid in terms of the Shariah is not permissible. However, if a person is liable to be arrested if he does not stand for the anthem, then standing will not be sinful for him due to the compulsion. But he should abhor it in his heart and recite Istighfaar.