Rights groups are calling on Ethiopia's reformist new leader to stop using repressive tactics once favoured by the previous leadership, in their most outspoken criticism since Prime Minister Abiy Ahmed took office in April.Human Rights Watch on Saturday highlighted the recent detention of hundreds of youth in the capital, Addis Ababa, after they rallied to condemn killings of ethnic minorities on the city's outskirts.Ethiopia on Tuesday released some 1 100 of the youth from a military camp after a monthlong detention in which the federal police commissioner said they were "brainwashed."Several hundred others are still detained for frequenting shisha cafes and dens where khat, a stimulant, is chewed.Human Rights Watch said the camps have no legal basis and their use "signals a worrying return to a period when the security forces frequently carried out arbitrary arrests".Returning to mass arrests puts Abiy's dramatic political reforms at risk and "Ethiopia could easily backslide", the group said.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and FacebookEarlier in the week, Amnesty International issued a statement criticising the arrest of a human rights defender, with researcher Fisseha Tekle saying that "instead ... Ethiopian police should work towards ensuring a safe and enabling environment in which defending human rights can be done without fear of reprisals."The detention of hundreds of youth has generated notable criticism of the new prime minister among Ethiopians for the first time, with some on social media calling him a "new leader in old clothes."The government has not responded to the statements. Abiy came to power after more than two years of anti-government protests spread across the country with demands for wider freedoms in Africa's second most populous nation.Since taking power, the new prime minister has announced a breathtaking series of reforms including the release of thousands of political prisoners, the removal of organizations from terror lists and the start of a process to rewrite once repressive laws.Now his government faces the challenge of balancing the new freedoms with keeping sometimes violent tensions among the country's scores of ethnic groups in check.