Tens of thousands of people marched through the centre of the Belarusian capital Minsk on Sunday to demand that authorities free political prisoners, prompting police to turn water cannon on them.

The march crowned a series of rallies in Belarus since an Aug. 9 election in which President Alexander Lukashenko claimed a landslide victory. His opponents say it was rigged to hand him a sixth term in power.

The crowd, waving white flags with a red stripe, marched to the beat of drums towards detention centres where political prisoners were believed to be held, according to pictures and videos posted on social media.

Belarusian police have used water cannon mounted on armoured police vehicles to disperse protesters and detained scores of protesters.

Armed policemen in black helmets holding batons cordoned off streets in central Minsk.

Mobile internet in Minsk was down from around midday and telegram channels, the main means of communication and distribution of photos and videos from the protests, asked residents of Minsk to unlock their Wi-Fi networks if protesters march nearby.

The political crisis in Belarus has caused broad repercussions for Minsk and its ally Moscow. This week, the European Union and the United States imposed sanctions on senior Belarusian officials, but spared Lukashenko.

The crisis has also raised the possibility of more sanctions against Moscow if it decides to offer a helping hand to Lukashenko. The rouble has already dropped steeply at the prospect.

Lukashenko denies electoral fraud and was inaugurated in late September in a ceremony held without any prior announcement, bringing more protests and condemnation from the EU, the United States and Britain.