Sayyiduna Ibn Abbas (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) said: "Any woman who marries without a guardian (wali), her marriage is void, her marriage is void. If he consummates the marriage with her, she is entitled to the dowry (mahr) in return for what he made lawful of her private parts. And if they dispute, then the ruler is the guardian of the one who has no guardian."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11446