Uganda's parliament has imposed a tax on the use of social media in a bid to raise revenue but opponents of the law say it aims to stifle criticism of President Yoweri Museveni, who has been in power since 1986.

Users will be charged 200 shillings ($0.0531) per day for services such as Facebook, Twitter and WhatsApp. That amounts to around $19 per year in a country where gross domestic product per capita was around $615 in 2016, according to World Bank figures.

The tax was passed on Wednesday as part of an overhaul of an excise duty law due to take effect next financial year which starts in July, parliament spokesman Chris Obore told Reuters.

A junior finance minister previously told journalists the tax would be levied daily by mobile phone operators on each SIM card used to access any of the targeted social media platforms.

There was no immediate comment from cell phone operators or social media companies, but rights advocates denounced the move.

"It (tax) is a new tool of stifling free expression and citizen organising that has been beyond the control of the state," said Nicholas Opiyo, a Kampala-based lawyer who also heads a local rights organisation.

"It's intended to curtail the ever increasing central role of social media in political organising," he said.

A government spokesman did not return a call seeking comment but authorities have previously denied such accusations.

About 40 percent of Uganda's 40 million people use the internet, according to data from the regulating body Uganda Communications Commission. Facebook and WhatsApp are widely used in Uganda and many other African countries.

Digital advocacy group the World Wide Web Foundation says data costs in Africa are among the world's highest, a fact blamed for slow internet penetration and limited use even for those connected.

"Data right now is essential in nearly every worker's day to day business, a responsible government should be lowering its price not the opposite," said Diana Taremwa, a charity worker in the capital Kampala.

Critics of Museveni, 73, say his government employs a wide array of tactics to limit political debate, trample on civil rights and stifle the opposition.

Museveni has won a series of elections but his opponents say these have been rigged in his favour. His main rival, Kizza Besigye, has been jailed dozens of times since he first run against him in 2001.

Some opposition critics have in the past been charged for allegedly insulting him in posts on Facebook.

In the last presidential election in 2016, authorities also blocked access to Facebook, Twitter and WhatsApp saying the platforms would be used by the opposition to mobilise protests.