The detention of four members of Taiwan's pro-reunification New Party has triggered an uproar on the island and the mainland, and the island's "ruling" Democratic Progressive Party has been accused of "political persecution".

Prosecutors in Taiwan took four young officials from the New Party away and searched their homes on Tuesday morning for alleged violations of the island's security regulations.

They were released on Wednesday morning.

Coincidently, three of them had visited the Chinese mainland last week in a delegation representing the New Party and met with top political adviser Yu Zhengsheng.

The Chinese mainland strongly condemned Taiwan authorities' suppression and persecution of pro-reunification entities and individuals on Tuesday night, said An Fengshan, spokesman for the Taiwan Affairs Office of the State Council.

An hailed the New Party's stance of upholding the one-China policy and opposing "Taiwan independence", and condemned Taiwan authorities for indulging the separatists while acting against those who support peaceful reunification.

He also said the Chinese mainland will closely follow the matter.

"It is harmful for the cross-Straits relations, under the current circumstances. The incident will have a negative influence," said Wang Hailing, a researcher of Taiwan studies at the Shanghai Academy of Social Sciences.

Wang also noted that it could be an act of political persecution, considering the timing and the fact that the four officials all publicly oppose "Taiwan independence" and support reunification.

"The timing is quite strange ... questioning and taking them away when they just returned from the mainland," he said.

Because Taiwan's "ruling" Democratic Progressive Party refuses to recognize the one-China policy, the official communication mechanism has been suspended since June 2016.

The incident might be a DPP move to gain face and express dissatisfaction, Wang said.

"But it was a bad ... move. The DPP chose good timing but lacked concrete and reliable evidence," Wang said.

Chiu Yi, a professor at Taiwan-based Chinese Culture University, wrote on social media that the "ruling party" in Taiwan has launched "green terror" with the purpose of suppressing the voice against "Taiwan independence". Green is the DPP flag's color.

"It is just a beginning - more dirty deeds are coming," he wrote.

The island's former "leader" Ma Ying-jeou also expressed dissatisfaction through his office.

He said doubts have been raised on the "green terror" and the actions were compared to "fascism". He hoped the departments concerned will publicize the legal basis and necessity of the search and questioning to dispel people's concerns.

Xinhua contributed to this story.