A marriage ceremony was conducted in a registrar’s office in a European country, where the bridegroom was a Muslim and the bride a Christian. The attending official was not a Muslim. Is this marriage valid from the Islamic point of view? If not, how can it be legalized?

From what I know about civil marriages in European countries, I tend to think that such marriages are perfectly valid if certain elements of Islamic marriage are observed. There are normally two witnesses in addition to the registrar and a clerk. The minimum requirement in Islam for a marriage contract is the presence of two witnesses. Any number of relatives and friends may also attend the ceremony at the registrar’s office.

According to Islam, the bride’s father or guardian should act on her behalf. The civil authorities in a European country expect a bride to act for herself. If her father or guardian is present, then the marriage contract is perfectly valid. In an Islamic marriage, a dowry should be mentioned as payable by the groom to his bride. This is not taken into account in a civil marriage in Europe. The amount of the dowry can be agreed upon between the man and his wife either before or after the marriage. The validity of the marriage is not affected.

As your question concerns the marriage of a Muslim man to a Christian woman, the requirement of a guardian can be waived.

Some scholars are of the view that the witnesses must be Muslim. But this is not a view to which all scholars subscribe. Many leading scholars do not stipulate this requirement. However, if the witnesses to the particular contract you have mentioned, or at least one of them, were Muslims, then the marriage contract lacks nothing. Perhaps I would add here that there is no requirement that the witnesses should sign the contract. This means that if there are a few relatives and friends, some of whom are Muslims at the time when the marriage contract is made, then the requirement of Muslim witnesses, which is stipulated by some scholars is considered to have been met. The contract is valid from the Islamic point of view even though the witnesses who sign the contract may not be Muslims.

In short, the marriage contract you are asking about is valid from the Islamic point of view and needs no further legalization. However, to be absolutely sure, the person concerned can go to the mosque or Islamic center in h/her country of residence and ask the imam or someone else there to go through the marriage contract. In certain European countries, you will find that the imam will insist on having a civil marriage first before solemnizing the marriage in the Islamic manner to comply with the requirements stipulated by the authorities in the country concerned.
And God Almighty knows best.