Pictures posted on various Internet sites showed servicemen on patrol in the trench, with the large dike looming above them.

''We have done this on our own and dug a trench practically the entire length of the border. In particularly dangerous places, concrete blocks standing on four legs have been put in place,'' Mr Taruta told a news conference. ''Our border is not a castle. But it is equipped so that vehicles cannot cross it in either direction. This is not based on one or another scenario, but rather intended to maintain a solid border.''

The trench, he said, was four metres wide, 2.5 metres deep and backed up by a dike two metres high.

Mr Taruta, appointed by the central government like all regional governors, said the aim of the trench project was to protect the 150 kilometres of the Donetsk region's border with Russia, particularly between established crossing points.

Donetsk is the home region of ousted president Viktor Yanukovych, removed last month after three months of sometimes violent protests against his decision, under Russian pressure, to abandon an agreement on close links with the European Union.

The government that came to power after his flight from Ukraine is firmly pro-Western. Moscow says it is linked to extreme right-wing groups which played a major role in the protests that led to Yanukovych's overthrow.

One demonstrator died in Donetsk last week after clashes between activists at rival rallies, one backing Russia and its takeover of Ukraine's Crimea region, the other Ukraine's drive to move closer to western Europe. Two activists died in unrest in Kharkiv, in northeastern Ukraine.

Mr Taruta said measures would be taken to ensure that pro-Russian protesters who have been staging rallies for weeks in Donestsk would no longer force their way into public buildings. On Sunday, a group of several thousand smashed their way into the local prosecutor's office to press demands for the release of a man who briefly proclaimed himself governor this month.

''This was a pre-planned action and they were trying to provoke us. We therefore did not try to stop them storming the building,'' he said. ''But as of tomorrow, the picture will be entirely different. The law will be respected and the security forces will be present. There will be changes in personnel.''