Saudi Arabia has intensified a crackdown on dissidents and rights activists after a respite that coincided with the country's G20 presidency last year, Amnesty International said Tuesday.

Saudi authorities "have brazenly intensified the persecution of human rights defenders and dissidents and stepped up executions over the past six months, following a lull... during Saudi Arabia's G20 presidency," the rights group said in a statement.

At least 13 people have been prosecuted, sentenced or had their sentences ratified "following grossly unfair trials before the Specialised Criminal Court", Amnesty said.

It said recorded executions fell by 85 percent last year, but that "at least 40 people" were put to death between January and July this year, "more than during the whole of 2020".

"As soon as the G20 spotlight on Saudi Arabia faded, the authorities resumed their ruthless pursuit of people who dare to express their opinions freely or criticise the government," Amnesty's Lynn Maalouf said.

In one instance, a humanitarian worker was sentenced to 20 years in prison "for a simple tweet in which he expressed criticism of economic policies", she added.

The release of several rights activists, including prominent women's activist Loujain al-Hathloul, has been "marred by restrictive conditions" including five-year travel bans, Amnesty said.

Since becoming crown prince in 2017, de facto Saudi ruler Mohammed bin Salman has pursued a liberalisation drive, spurred by the need to diversify the Gulf country's oil-reliant economy.

But simultaneously, the prince has launched a sweeping crackdown on dissent and free speech.

Rights organisations and activists regularly accuse the regional powerhouse of violating human rights and targeting dissidents, including journalists and women's activists.