A second protester has died in Belarus in police custody following violent clashes with police that have been ongoing since Sunday’s disputed presidential election, which has been denounced as rigged by international observers. Officials said the cause of death of the 25-year-old man in Gomel isn’t clear—but his mother said he had heart problems and was locked inside a police van for hours. Across Belarus, police officers were reported to have used excessive force, rubber bullets, water cannons, and stun grenades. The United Nations says that at least “6,000 people have been detained in the last three days, including bystanders, as well as minors, suggesting a trend of massive arrests in clear violation of international human-rights standards.” At least 200 protesters are known to have been wounded, some seriously, and journalists have also been attacked. A demonstrator died during a protest in the capital Minsk on Monday.