Kenya's main opposition coalition said that one of its offices in a Nairobi suburb was raided by police who made off with dozens of computers, servers and surveillance equipment.
The incident comes as the country is on a knife edge just days ahead of general elections next Tuesday.
An AFP reporter at the scene of the raid at the office in the Westlands suburb saw dismantled cables, a broken door and chairs and tables turned upside down.
"What we are witnessing is a very unusual situation in the sense that one of our centres, our tallying centres was raided in the course of the evening," Musalia Mudavadi, one of the leaders of the National Super Alliance (NASA) told reporters at the scene on Friday.
"They damaged the place and disappeared … with all the desktops, some laptops as well."
Earlier, a Nasa statement said that "police in hoods and armed with AK47 and machine guns raided different offices" belonging to the coalition.
However, no other raid at any other office belonging to the coalition could be confirmed.
A senior police officer denied any knowledge of the raid.
A guard at the affected building, speaking on condition of anonymity, said: "They were all armed and when they walked in they introduced themselves as policemen and were brandishing guns, others were left outside while others went in.
"They had covered their faces and from the way they spoke they were very confident and were not in a hurry."
Mr Musavadi said "we have no doubt this is the work of the government. It is unfortunate they have resorted to this, but I can assure you that this will not deter us at all."
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Tensions rise as Kenya poll campaign enters final stretch
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Polls show Nasa's flag-bearer Raila Odinga in a dead heat with longtime rival president Uhuru Kenyatta and tensions are rising.
Mr Odinga says that the only way he will lose is if the poll is rigged. It is his fourth shot at the presidency and he claims elections in 2007 and 2013 were stolen from him.
Nasa has locked horns with the national election commission (IEBC) with its plans to set up a parallel tallying centre to announce its own results.
Mr Kenyatta's ruling Jubilee party denounced this as a plot to dispute the results.
The IEBC eventually agreed to the opposition plan, as long as it did not announce its own final result.
The country was also stunned by the murder and torture of a top IT official at the national election commission, who oversaw an electronic voting system seen as crucial to the success of the poll.
He was found strangled in a forest on the outskirts of Nairobi last weekend.
While 2013 polls were largely peaceful, it is a decade since a disputed 2007 presidential election led to two months of violence, leaving 1,100 dead and 600,000 displaced.