Image copyright Roland Hunziker

Visionaries hope for a modern metropolis modelled on Singapore, but pessimists fear the emergence of another dirt-poor city of slums. Dar es Salaam is one of the world's fastest growing cities, and it has reached its tipping point.

In the dark basement of the cavernous Kariakoo market, dozens of traders gather at tiny makeshift stalls, arranging fruit and vegetables into neat piles. This part of the market has the least sought-after plots, and all of the stallholders have one thing in common: none of them was born in Dar es Salaam.

Rolens Elias arrived seven years ago from a village near Morogoro, about 150km to the west. He had been a farmer but wanted to try his luck as a trader. He now makes about 3,000 shillings ($2; £1.50) each day selling tomatoes in the farthest corner of the basement.

"It has been hard to set up a life here," he says. "I came here by myself and had to wait until I had enough money to bring my wife and family. We all live in one room, but it's a better life than in the village."

As he arranges his tomatoes, a group of his friends gather around and chip in with their own stories. They are all from Morogoro, and all came to Dar es Salaam in the hope of a better life. They all contrast the rural poverty they were born into with the lure of Dar es Salaam and its big-city opportunities.

Their stories are repeated many thousands of times across the city.

Every day new arrivals flood in, many of them setting up home in hastily erected shacks, many others sleeping on the streets. Those who cannot set up as greengrocers hawk goods, anything from baseball caps and mobile-phone chargers to bottled water and sweeping brushes.

The dramatic influx has pushed the city's population up from roughly two million two decades ago to four million today. If the government does nothing, the population will hit eight million in 20 years, according to Nimrod Mushi, a lecturer at the city's Ardhi university.

He is one of the experts commissioned by the government to produce a "master plan" to overhaul the city's infrastructure. Singapore is his role model, and he favours big projects to clear slums and build bridges, roads and out-of-town settlements.

"When we went to Singapore, we could see their satellite towns, their ring-roads, their skyscrapers and their decentralised services, and it's working very nicely there," he says.

He points out that Dar es Salaam has gone 20 years without any guidance on planning, and now "badly needs a master plan".

Superficially at least, his dream seems within reach. Tanzania's economy is booming, and cranes litter the skyline, putting the final touches to high-rise blocks. The country's super-rich, many of whom make their cash in the far-away mines of the north, are pouring money into the city.

But Dar es Salaam is a long way from Singapore. The Asia city-state's economy was worth $260bn last year compared with $23bn for the whole of Tanzania, which remains one of the poorest countries in the world.

The UN estimates that 70% of Dar es Salaam's population live in informal settlements; there are no slums in Singapore.

Slum clearance would be vital to any regeneration project. It would involve rehousing possibly hundreds of thousands of people, and the extra headache of clarifying the legal status of the land that has often passed down through generations of families without any legal paperwork.

The Tanzanian authorities do not have a great record with such projects. Some 600 families had their homes destroyed by floods late last year. The government promised to rehouse them, but most are still living in temporary shelters or have been left to fend for themselves.

Such stories point to a deeper problem with Dar es Salaam's development.

In pictures: Life in Dar es Salaam

The divisions in Tanzania's lop-sided economy, with a tiny super-rich elite and a vast poor majority, are reflected in its main city. The poorer residents crowd into dilapidated downtown areas or sprawling slums, many without running water or basic services. Their rich counterparts can choose between $1m beachside mansions in the city's posh northern districts.

Among many of the city's poorer residents, there is profound resentment at the authorities.

Image copyright Getty Images Image caption A royal visit from the Duchess of Cornwall and Prince Charles in 2011 was popular

"Every day I pay 700 shillings to the government for my stall," says avocado seller Mzeze Chouheke Gady. "If I make 5,000 shillings, I pay 700. If I make nothing, I still pay 700. What do I get in return?"

His gripes include a lack of medical care, transport or basic sanitation. Many other people share his grumbles.

Nimrod Mushi warns that a planning overhaul is the only way to solve these problems. But he concedes that its success or failure depends on financing.

"When the document comes out later this year, we will include all the budgets needed," he says.

"Then it's up to the government, or a donor government, to allocate resources. Otherwise it'll be just another book on the shelf."

Rakesh Rajani, who runs a development project in the city, is also worried about the way the authorities are handling the city's growth.

Image caption Informal settlements line the roads into the city

He says the influx provides an opportunity to harness the talents of the people and create a great city, but worries that the authorities too often oppress the poor with petty rules and rough treatment.

He says Tanzania could face a similar conflagration to Kenya in 2007, when thousands of people were killed in post-election violence.

His analysis runs against the popularly held view of Dar es Salaam as a beacon of diversity, where people of different religions and ethnicities live together peacefully.

But Rajani is more plugged in than most to the collective mind of the city. His project, called Twaweza, runs regular phone-based polls on major issues aiming to reflect the views of Dar es Salaam's residents.

"We live on that knife-edge," he says.

"Sadly I think that at times the people in charge are squandering this opportunity, and possibly mismanaging things to the extent that it could turn very nasty."