The new constitution, which would give more power to Bolivia's indigenous majority, has been the subject of violent protests by the country's opposition.
Morales had joined the last stage of the week-long 200km march in favour of the constitution by peasant farmers, activists and miners as it entered La Paz.
The new constitution would allow him to seek re-election in 2009 but forbids him from seeking a third term in 2014.
Violence broke out over the proposals last month, leaving 19 people dead and martial law imposed by the goverment in one province.
Bolivia's constitutional court had rejected a decree Morales had issued calling for a December referendum on his new constitution, saying such a move could only be scheduled by congress.
The move led to the protests by rural workers and union members backing Morales.
The proposed constitution would redistribute land to Bolivia's poor indigenous communities, but has met fierce resistance from provinces in the east of the country, who say it overlooks their demands for greater regional autonomy.