The rematch sold 20,000 tickets in the first five minutes. Then the website crashed, promoter Eddie Hearn told ITV News' Rags Martel.
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The 48-year-old man was held on suspicion of theft from heritage and protected sites.
A Met Police officer was left red-faced after attempting to stop a cyclist riding along a dedicated cycle path.
The number of people allowed into official spectator areas will be cut by a fifth with just 100,000 tickets available.