Nigeria's government has told the country's mobile phone networks to block access to Twitter, a statement from the industry umbrella body said.
Phone users are reporting difficulties in accessing the site. It is still available through some wi-fi networks.
This comes after the government said it was suspending Twitter operations in the country "indefinitely".
It alleged the site was being used to undermine "Nigeria's corporate existence".
Twitter has called the ban "deeply concerning".
The Association of Licensed Telecommunications Operators of Nigeria (Alton) said its members complied with the request to suspend Twitter access based on "national interest provisions" in the telecoms law and the licensing terms.