Vietnam's Communist Party delegates are meeting in Hanoi to begin the process of choosing a new set of leaders.
The congress, which happens once every five years in the one-party state, will run for eight days.
Behind closed doors, the 1,510 members will nominate the next president, the prime minister, and most importantly, the party's general secretary.
Who wins could determine Vietnam's relations with key partners including the US and China.
Vietnam's closely controlled media are not allowed to cover the congress except for the opening ceremony.
On Thursday, top leaders gave brief speeches emphasising the need to build Vietnam's economic prosperity, and also reviewed the leadership of the central committee, reported Viet Nam News.
Nga Pham, BBC Vietnamese: 'Rumours and speculation'
As most important meetings of the top echelon of the party are conducted behind closed doors and little information is ever made public, it is inevitable that rumours spread widely ahead of any congress.
But Vietnam has never seen the level of speculation circulating this time. Right up to the opening day of the congress there are still divided opinions about who is going to become the party's general secretary on 28 January.
Party officials themselves admitted that at the last three-day meeting of the powerful central committee before the congress, participants had to hold a staggering 13 rounds of voting to come up with a list of nominees.
But whoever is elected, the delegates currently meeting in Hanoi's My Dinh National Convention Centre are determined to make sure that the country will continue to tread the "Socialist path", with the Communist Party secure in its rule over the nation of 94 million people.
The competition for the general secretary job pits current Prime Minister Nguyen Tan Dung, seen as modern and friendly towards the US, against party stalwart and incumbent, Nguyen Phu Trong.
Mr Dung has gained popularity domestically with strong anti-China rhetoric when it comes to disputed territory in the South China Sea.
Mr Trong is seen to be more friendly towards China - Vietnam's largest trading partner.
The congress takes place amid resurgent tension with China. On Tuesday, Vietnam said it had demanded China remove a controversial oil rig parked in waters claimed by both countries.
In 2014, Beijing moved the same rig into contested waters, sparking violent anti-Chinese protests in Vietnam.
How new leaders will be chosen:
- Delegates will elect a new central committee.
- On one of the last days of the congress, the new central committee will meet to select a 16-member politburo from among its ranks.
- From within the politburo a general secretary, prime minister and president will be nominated.
- The general secretary will need approval from congress.
- The prime minister and president will be approved by the national assembly in about six months time - however this is normally just a rubber stamp.