Zimbabwe’s constitutional court has rejected an opposition challenge to Emmerson Mnangagwa’s election victory last month.

The decision was widely expected, but is unlikely to end the former British colony’s political instability.

Opposition officials have said they will continue to put pressure on the government by any other legal means.

Luke Malaba, the chief justice, gave the Harare court’s verdict after considering arguments put before judges earlier in the week.

Lawyers for the opposition Movement for Democratic Change (MDC) argued the results of the 30 July election should be annulled because the electoral commission had collaborated in fraud.

“There is a massive cover-up. There has been a massive doctoring of evidence,” Thabani Mpofu, representing the MDC, told the court on Wednesday.

However, Malaba said the court could only invalidate a presidential election in very limited and specific circumstances, and the MDC had not provided specific evidence of the “general” allegations they had made.

Douglas Mwonzora, the MDC secretary general, said the party would respect the decision of the highest court of the land and had not yet made a decision about a campaign of protests.

“As far as the legal challenge [is concerned], this is the end of it …… There are other means to resolve this dispute, but the MDC wants to make it clear that it will act within the confines of the law,” he said. “It is still too early to say because the national executive is going to meet next Wednesday to discuss a concrete way forward.”

Riot police with water cannon on the streets of Harare. Security was tight in the capital ahead of the verdict. Photograph: Tsvangirayi Mukwazhi/AP

The election was the first since Robert Mugabe, now 94, was ousted in November after 37 years in power.

Mnangagwa said on Twitter: “I once again reiterate my call for peace and unity above all. Nelson Chamisa, my door is open and my arms are outstretched, we are one nation, and we must put our nation first. Let us all now put our differences behind us. It is time to move forward together.”

The relative peace during campaigning contrasted with the systematic violence that marked elections in previous years, and raised hopes of a new and more tolerant era in Zimbabwe.

Mnangagwa, a 75-year-old former spy chief and ruling Zanu-PF party stalwart, won with 50.8% of the vote, narrowly avoiding a runoff with the 40-year-old MDC leader, Nelson Chamisa.

Chamisa, a former pastor and lawyer, who won 44.3% of the vote, according to election officials, has repeatedly described the poll as “fraudulent and illegitimate”.

Lawyers for the government dismissed claims the opposition had produced any evidence of fraud, accusing the MDC of being “flippant”.

Nine judges, led by Malaba, heard the case in Harare, with tight security around the court. The atmosphere in the capital was tense on Friday, with a heavy police presence and helicopters overhead.

In a first for Zimbabwe, the proceedings were broadcast live on state television.

Mangagwa needs international legitimacy to obtain the multibillion-dollar bailouts required to avoid economic breakdown. The president, long seen as Mugabe’s political vehicle, has made efforts to convince observers he is a reformer.

International monitors largely praised the conduct of the election, although EU observers said Mnangagwabenefited from an “un-level playing field”.

Six people were shot dead when soldiers opened fire on opposition demonstrators in the centre of Harare two days after the vote, and scores were subsequently arrested by security agencies. Many were badly beaten. Among those detained were senior MDC leaders who have been accused of inciting violence.

The party cited a catalogue of discrepancies including incorrect counting, “ghost” polling stations and, at some polling stations, more ballots being counted than there were registered voters.

The EU and its member states issued a joint statement within minutes of the court judgment, expressing concern at recent violence and calling for calm, respect for the constitution and inclusive politics.

Derek Matyszak, a legal expert at the University of Zimbabwe, said the opposition faced an uphill struggle given the courts’ historic tilt towards Zanu-PF, which has ruled since independence from British rule.