Venezuelan authorities ordered the arrest of a close aide to opposition leader Henrique Capriles and military agents removed documents, computers and cellphones from the man’s apartment on Wednesday, the opposition said.

It called the action proof of a new wave of political repression.

Venezuela’s highest court, meanwhile, rejected Mr. Capriles’ bid to void the April 14 presidential election, which he narrowly lost to ruling party candidate Niclas Maduro. It also fined Mr. Capriles $1,700 for what it called a “disrespectful” filing and asked the attorney general to prosecute the opposition leader for filing the petition.

The 12-party MUD opposition coalition did not elaborate on the government’s stated reason for issuing the arrest warrant for Oscar Lopez, the chief of staff to Mr. Capriles in the Miranda state governor’s office.

Mr. Maduro had announced earlier in the day that the government “today captured a chief of the corruption and of the mafias of the Venezuelan right.” He did not identify the person but said “he was caught red-handed.”

The Associated Press telephoned the chief prosecutor’s office but it offered no comment.

Mr. Lopez’s lawyer, Leonardo Palacios, told reporters that soldiers removed personal documents, computers and cellphones from his apartment Wednesday morning. He said a judicial order had sought bank account documents without specifying the nature of the investigation.

Foreign Minister Elias Jaua suggested via Twitter that the probe was related to the financing of Mr. Capriles two presidential campaigns. In October, Capriles lost to long-time President Hugo Chavez, who died in March of cancer.

MUD, in the statement announcing the arrest order, called it part of “a new attack against those who don’t stop fighting for the restitution of legality, justice and rights in Venezuela.”

It charged that the arrest order was part of a strategy to divert attention from Venezuela’s problems, among them rampant crime that includes one of the world’s top five homicide rates according to the U.N. and crippling inflation that the government said this week is running at an annual rate of 42.6 percent.

Also Wednesday, Venezuela’s Supreme Court threw out Mr. Capriles’ challenge to the election results. Mr. Capriles claims Mr. Maduro secured the 1.5-point victory through fraud after squandering a double-digit lead in the polls.

The court declared was “unacceptable” for Mr. Capriles to seek the invalidation of the entire vote. It fined him 10,700 bolivars for the “offensive content” and “disrespectful terms” of his court filing and said prosecutors should investigate whether Mr. Capriles was criminally liable for trying to undermine the credibility of the election.

Mr. Capriles on Wednesday called Mr. Maduro “a huge coward” on Twitter for going after his aide and said “millions and millions will run you out of Miraflores (Palace). Nothing will save you from when we apply the Constitution!”

In recent weeks, the government has stepped up pressure on several key opposition members.

It announced its intention to freeze the bank accounts of the Miguel Henrique Otero, publisher of El Nacional, one of Venezuela’s two remaining nationally circulated newspapers.

Authorities jailed and froze the assets of the editor of a small independent paper and website, Leocenis Garcia of Sexto Poder, who has published detailed reports of alleged government corruption, including what he calls multimillion—dollar graft involving manipulation of currency controls.

And the National Assembly, controlled by allies of Mr. Maduro, stripped opposition lawmaker Richard Mardo of immunity from prosecution, opening the way for the government to pursue a tax fraud case against him. The opposition calls it a trumped-up attempt to silence a critic.

The late July vote by lawmakers drew especially vehement reproach from the opposition because the Constitution specifies that a two-thirds vote is required in the National Assembly to strip a deputy of immunity and the ruling socialists fell 12 votes short of that threshold in the 165-seat chamber.

The opposition and international human rights groups accuse Maduro of trying to crush legitimate, law-abiding dissent by systematically flaunting the socialist administration’s control of all branches of government secured under Chavez.

Mr. Maduro accuses the opposition of conspiring with foreign agents led by the United States to overthrow a democratically elected government.

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