MONTEVIDEO, Uruguay (AFP) - Uruguay is to give the green light on Tuesday to making marijuana legal, in a social experiment that countries plagued by drug-related crime worldwide will watch.

The legal change in this largely rural and well-educated South American nation of just 3.3 million looms large with a vote in the Senate.

The Lower House of Congress passed the Bill in August, so passage in the Upper House is assured because the ruling leftist Broad Front coalition controls both.

"It is going to be a history-making day. This puts Uruguay squarely in the forefront of changing policymaking and strategies," said Senator Luis Gallo of the Broad Front.

The plan was unveiled a year and a half ago by President Jose Mujica along with other measures aimed at ending crime and violence associated with the drug trade.

"This is an experiment", Mr Mujica, 78 and a doctor by training, told Agence France-Presse in August.

"We can make a real contribution to humanity. Be a testing ground with a series of measures to confront the problem and provide tools to fight drug addiction," he said.

The law will give the government control and regulatory power over imports, growing, harvesting, distribution and sales of pot and its derivatives.

After signing up, people 18 and older will be able to grow up to six marijuana plants, get the drug in marijuana smoking clubs and buy up to 40g a month in pharmacies.

Uruguay's change on pot comes in addition to other significant moves in the country with a leftist government: legalisation of abortion, gay marriage and same-sex adoption.

But not everyone is thrilled by the idea of legalising marijuana. A poll carried out in September found a high 61 per cent of those surveyed do not approve.

Continental heavyweights and next-door neighbours Brazil and Argentina have expressed surprise. Uruguay long has been a main beach destination for Argentinians, and tourism is important for Uruguay but not its only industry.

Uruguay also is a key agricultural exporter and increasingly popular outpost with service industries.

Many analysts wonder what would happen if such a law were passed in countries torn and bloodied by drug trafficking, such as Colombia and Mexico.

Uruguay frames the change as supporting an international drug panel whose members include former presidents Cesar Gaviria of Colombia and Ernesto Zedillo of Mexico and which has concluded that the US-led "war on drugs" has failed.

Currently, consuming drugs is not illegal in Uruguay but selling illegal drugs is. Pot is the most common of illegal drugs here and consumption of it has doubled in the past 10 years.

The authorities say there are some 128,000 marijuana smokers in this country, but pot consumer groups but the figure higher, at 200,000.

Uruguayan doctors are divided. Some say the change will lower consumption of more dangerous drugs, while others argue that it ignores the pernicious effects of smoking pot.

Opposition lawmakers say there is the risk that consumption will rise.

Experts on the global drug trade and social impacts of illegal drug use are divided on Uruguay's legal pot outlook as well.

Mr Arjan Roskam, head of Holland's Green House, the world's biggest marijuana seed bank, said that if the regulation is done "in a very socialist way it will not work".

Mr Roskam, one of the experts consulted by the government of Uruguay, told Agence France-Presse that if everything depends on the state and sales to tourists are not allowed, there will still be street dealing and the black market problem will not be solved.

"So you do something very good, you regulate cannabis, but you do not solve the dealing in the streets," he said.

Still, many local marijuana users are ready to light up with a smile. One last march against illegal pot is planned.