The National Freedom Council (CNL) spokesperson Terence Manirambona says the arrests continued into Thursday.
“Most of those arrested were CNL observers at polling stations, it happened in every province of the country, we could see it was a hunt on them,” he told BBC Great Lakes.
Police have not commented on the arrests but earlier a spokesperson said general election day had been generally peaceful.
CNL’s leader and presidential candidate Agathon Rwasa is considered the main challenger to the ruling party’s Evariste Ndayishimiye.
“Our observers, any who could tell poll officers that ‘what you are doing is against the law’ would be accused of disrupting the poll and thrown to jail,” Mr Manirambona said.
Some of those arrested arrested had gone to observe vote counting on Thursday, he said.
Some had already been arraigned in court for what Mr Manirambona termed as “non-factual charges”.
Meanwhile, social media users in Burundi were able to reconnect to Twitter, WhatsApp and Facebook on Thursday, ending an outage that started on Wednesday morning.
President Pierre Nkurunziza is stepping down following 15 years in power, during which his government has repeatedly been accused of serious human rights abuses.
SOURCE: BBC WEBSITE