Spanish term (edited): Asistí al matrimonio que por palabra de presentes contrajeron P y Q.

I attended the ceremony that, as attested by those indicated herein, joined P and Q in holy matrimony.

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"Palabra de presentes" seems here to refer to the witnesses who have signed the certificate.

This is a free translation translation that sounds natural in English. There are, of course, other possibilities, but I don't see where a strictly literal rendering would work here.

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Note added at 22 mins (2021-09-14 11:16:20 GMT)
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ERRATUM:

I meant to write the first sentence above as follows:

"Palabra de presentes" seems here to refer to the participants and witnesses who have signed the certificate.