As Vietnam's ruling Communist Party gears up for its most important meeting in years, its leadership has presided over an intensified crackdown on dissent, according to rights groups, activists and data collated by Reuters.

A record number of political prisoners, longer jail terms, and increased harassment of activists in recent years have contributed to the crackdown ahead of this month's Communist Party congress, a gathering to determine national leadership and policy that takes place once every five years.

The crackdown has left some international human rights groups and lawmakers questioning whether Vietnam has breached the spirit of trade agreements with Western countries – accords that have helped propel the country to a position of economic strength in Southeast Asia.