Foreign observers deployed to oversee Kenya's General Election called Thursday for patience while votes are counted as accusations by the opposition of rigging heightened tensions in the country.

Opposition candidate Raila Odinga, who is trailing behind incumbent Uhuru Kenyatta, on Wednesday claimed massive hacking of an electronic system for sending votes to a national tallying centre in Nairobi.

Former US Secretary of State John Kerry, leading an observer team from the Carter Centre, expressed confidence in the integrity of the system, introduced in a bid to avoid fraud in a country prone to post-poll violence.

"We believe the IEBC (election commission) put in place a detailed, transparent process of voting, counting, reporting and securing the vote, all of which lends significant credibility and accountability," Mr Kerry told a press conference.

Polling officers have been sending results electronically to Nairobi, which are showing up in real time on a public website, but these need to be backed up with forms signed by them and party agents in each of the 40,883 polling stations.

READ: What are Forms 34A and 34B?

Mr Odinga, aside from claiming the electronic results are a "sham", has complained about delays in the sending through of the forms, further fuelling suspicion about manipulation.

Mr Kerry said his observer team had witnessed party agents counting ballots together, and deciding together which were valid or not before signing off on final tallies.

"All of this provides an extensive traceable trail of agreement by many parties on the paper balloting process and therefore on the outcome," he said.

Credible polls

Some 400 international observers were present for Tuesday's vote, and their team leaders from the African Union, Great Lakes region, Commonwealth and European Union while saying the voting process was credible, called for calm and restraint.

EU's Marietje Schaake said Thursday that they had seen no signs of "centralised or localised manipulation" of the voting process.

She said the IEBC was "working around the clock".

"It's important they have the time to do these procedures well. We continue to urge everyone to be calm, to be resilient and to be peaceful," she added.

Ghanaian former president John Mahama, who is leading the Commonwealth delegation, also urged Kenyans to give the IEBC "proper time and space to complete the results process with necessary due diligence.

"It is vital that all political leaders maintain peace and calm, exercise patience and not stampede the process," he added.

Mr Mahama noted that the "opening, voting, closing and counting process at polling stations were credible, transparent and inclusive".

South Africa's former president Thabo Mbeki, at the head of the African Union mission, agreed. He said the opposition's hacking complaints should be looked into by the IEBC, saying this was not the mandate of observers.

General Hassan Bushra of Sudan, heading the International Conference of the Great Lakes Region (ICGLR) mission, said he hopes the final result will reflect the will of the people.