Several thousand taxis have snarled morning rush-hour traffic in the Portuguese capital in the latest European protest against ride-hailing apps.

Portuguese taxi drivers complain that companies such as Uber and Cabify are not covered by the same tax, training and safety regulations as they are. They want the government to adopt specific legislation for ride-hailing services.

Two labor groups representing Portugal's around 13,000 taxi drivers organized the long line of slow-moving taxis that snaked through Lisbon on Monday in the second such protest in six months.

Hundreds of police were on standby in case of violence, and sporadic scuffles broke out as taxi drivers confronted vehicles they claimed were working with Uber.

Other European Union countries have also witnessed demonstrations by traditional taxi drivers against the increasingly popular ride-hailing services.