LONDON—A Hong Kong pro-democracy website was temporarily taken down after police warned the Israel-based hosting company that it breached a national security law, highlighting concerns about actions by authorities in the Chinese city to muzzle online dissent abroad. Nathan Law, a Hong Kong opposition leader based in the U.K., tweeted Thursday that the hosting company, Wix.com, received a request from the Hong Kong police department to disable the 2021 Hong Kong Charter website. He posted screenshots of the police notice telling the hosting company the site contained messages “likely to constitute offences endangering national security” and that it would be prosecuted if it didn’t comply. Wix, headquartered in Tel Aviv, said the website was removed by mistake and has been reinstated. “We have reviewed our initial screening and have realized that the website never should have been removed and we would like to apologize,” the company said by email. “We …