SINGAPORE—China has ordered its largest internet companies to stop blocking links from their rivals on their apps, as Beijing seeks to boost competition in a sector dominated by a handful of companies. Authorities have made the move against blocking external links a key focus of a campaign that started in July, after receiving multiple complaints from users, an official with the Ministry of Industry and Information Technology said Monday. Companies such as Tencent Holdings Ltd. TCEHY -1.76% , Alibaba Group Holding Ltd. BABA -1.47% , ByteDance Ltd. and Baidu Inc. BIDU -0.27% were among those called to a meeting last...