The security situation in the Democratic Republic of Congo is deteriorating ahead of elections on 28 November, human rights groups warn.
In a joint statement, the 40 groups said recent clashes between the police and opposition showed the potential for destabilisation.
Official campaigning for the elections kicked off on Friday.
The 2006 poll, won by President Joseph Kabila, was marred by widespread violence.
Mr Kabila has ruled DR Congo since 2001, following the assassination of his father, Laurent, and signed a peace deal the next year with rebels in the east to end a brutal five-year conflict.
In their statement, the 40 groups - which range from the International Crisis Group (ICG) to the respected Congolese think tank, the Pole Institute - said the UN mission in DR Congo should deploy a rapid reaction force to potential flashpoints of violence.
They said donor countries had poured billions of dollars into DR Congo, and could not afford a fraudulent election that could trigger a fresh wave of violence.
"This election in [DR] Congo is the ultimate test. Is [DR] Congo on course to consolidate its fledgling democracy or return to a state of widespread instability, insecurity and violence?" Thierry Vircoulon, the ICG's Central Africa director said.
The statement said recent clashes between the police and opposition - which left seven people dead in the capital, Kinshasa - showed the alarming potential for destabilisation.
There has also been an increase in violence against aid workers, with five of them killed earlier this month in the volatile east - the deadliest attack of its kind in DR Congo's history, the groups said.
They called on the police to refrain from using excessive force and political parties from engaging in hate speech during campaigning.
The elections are likely to be fiercely contested between the main opposition leader, Etienne Tshisekedi, who is standing for the first time, and President Kabila.
Mr Tshisekedi boycotted the 2006 poll, saying they had been rigged.
There have already been political disputes this time around about the voters roll, with the opposition saying false names have been added to it.
New election rules
BBC World Affairs correspondent Mark Doyle says the sheer scale of the logistics involved in preparing for the polls is also daunting.
DR Congo is a vast country with a poor transport network, and most election material will have to be transported by air, he says.
The ballot papers alone are so large and contain so many photos of presidential and parliamentary hopefuls that they weigh a total of 4,000 tonnes, our correspondent says.
There are 11 presidential candidates and nearly 19,000 candidates running for 500 parliamentary seats.
The electoral commission, helped by the UN peacekeeping force known as Monusco, will be responsible for distributing 186,000 voting boxes and 64 million voting cards to 62,000 polling stations, the AFP news agency reports.
Earlier this month, police used tear gas to disperse hundreds of his supporters who marched on the electoral commission building in protest at the alleged fraud.
Opposition parties have also criticised changes to the polling system aimed at electing the president in one round, which they say favour Mr Kabila.
The government says it only wants to avoid a repeat of clashes that marred the run-off in 2006.
The recent violence has been attributed to tension between Mr Tshisekedi's party, the Union for Democracy and Social Progress and Mr Kabila's People's Party for Reconstruction and Democracy.
The presidential elections in 2006 were the first democratic polls in DR Congo for four decades.