The grounds
Under article 49 of the Family Code: the person has not reached the marriage age; the person is in another registered marriage; close kinship; incapacity because of a mental disorder; coercion into the marriage; a fictitious marriage (a marriage registered with no intention to create a family); concealment of a venereal disease or HIV from the other person (if that person applies to the court).
The difference from divorce
Divorce ends the marriage for the future. A marriage that a court recognises as invalid counts as invalid from the moment of its conclusion (article 55). A marriage is recognised as invalid only through court procedure (article 50).
The rights of the children
The recognition of a marriage as invalid does not affect the rights of children born in that marriage, or born within three hundred days after that date.
The good-faith spouse
For a spouse who did not know that the marriage was invalid (the good-faith spouse), the court can recognise the right to maintenance and can divide the property acquired together by the rules on the common property of spouses. The good-faith spouse can demand compensation of material and moral harm, and can keep the surname chosen at the registration of the marriage (article 56).