A ban on plastic bags came into force in Kenya on Monday in a bid to slow pollution, with offenders liable to jail time or hefty fines.

The ban on the use, manufacture and importation of plastic carrier bags was carried through after the High Court threw out a challenge brought by importers who claim jobs will be lost and livelihoods threatened.

Dozens of countries have either restricted, banned outright or imposed levies for the use of plastic bags but Kenya's new law is particularly tough, with fines of up to $38,000 (32,000 euros) and four-year prison sentences.

The UN Environment Programme (UNEP) estimates that Kenyan supermarkets hand out as many as 100 million plastic bags every year.

UNEP head Erik Solheim greeted the ban as "a huge, important step" to ending plastic pollution.

- Confusion and queues -

The ban was first announced in February but suspended for six months to allow Kenyan consumers and shopkeepers to adjust to the new rules. 

Kenya's National Environment Management Agency (NEMA) took out ads in newspapers clarifying that while plastic carrier bags were banned, neither industrial manufacturers producing plastic-wrapped goods nor users of plastic bin liners will fall foul of the law.

In Kenya, road verges are commonly covered with discarded plastic bags and trees festooned with them, they block drains and are ingested by animals, including livestock such as cows and goats.

Environmentalists say the damage is worse still at sea where island-sized gyres of garbage float about and fish starve to death, their stomachs filled with plastic waste.

The Retail Trade Association of Kenya said supermarket chains plan to provide re-useable, eco-friendly bags at a small price.

"We are subsidising the cost for the benefit of the consumer," said Willy Kimani, director of the trade group and an executive at the Naivas supermarket chain.

Hours after the ban took effect on Monday there was confusion and long queues at supermarkets where shoppers were forced to carry goods in boxes or in their arms as the piles of plastic bags that used to hang at the end of checkout counters disappeared overnight.

Some Kenyans took to social media sites to complain of overzealous police stopping vehicles in downtown Nairobi and searching them for plastic bags and, they alleged, bribes.

This is the third time in a decade that Kenya has tried to impose a plastic bag ban but they are such a visible blight that the ban has widespread support despite the disruption.