The independent republic of India was officially born on the 26th  January 1950, after 89 years of British rule. However republican values have  been home to Indians since 2000 BCE. While the President of India gets a 21 -  gun-salute on the 26th of January every year, it wouldn’t be unceremonious to  give a similar gun salute to all those erstwhile republic states and  confederations that existed in ancient India. The republics of Ancient India  clearly show that India practiced good old republican values in the form of  ‘Gana rajyas’ and the ancient Indian society was a ‘Gana tantra’ at its best.
A republic is one where absolute  power does not lie with one individual or a monarch. In a republic there is  "liberty and justice for all" irrespective of a minority. The Vedic  literature, Jain and Buddhist texts show to us that ancient India was ruled by  councils of thinkers and learned men. We even find instances of Kings ruling  under a republican influence guaranteeing justice to all. Though India had  numerous kings and queens, they did not have absolute power as a monarch would.  Not all Kings were from a royal blood-line or ruled by divine right. Many were  even elected Kings.
Why we forgot about the old Indian republics…
  Because of the political slavery  that started with the invasion of India by the Mughal King Babur in 1526 CE  till the end of British imperialism in 1947, Indians forgot where they came  from. Historical research has finally uncovered that ancient India was a  knowledge society and had republics since ancient times. Another reason for  this historical amnesia about republics in ancient India is the thought that  republicanism started only after the French revolution when monarchy was given  a serious toss. 
A number of states in ancient  India had republican forms of government. Between 2000 BCE and 400 BCE a number  of republics flourished in India's river based civilisation especially in the  Gangetic plains of present day Uttar Pradesh and Bihar.
So where do we get this information about the old republics in India? 
  The republican forms of  government are known to us mainly from the surviving Buddhist and Jain  religious texts, Greek sources, Sanskrit epics and ancient text books on  politics. Greek writers about India such as Megasthenes and Arrian describe  many of the states having republican governments like those of Greece. These  ancient texts refer to a number of states having Gana sangha, or council-based  governance, as opposed to monarchies. 
Some examples of old Indian republics…
We learn about the rise of  republics prior to the rise of the great Gautama Buddha. We had a Videhan  republic, Vajjian republic etc from the ancient texts. Another oft-mentioned  example is of the sixteen Maha-janapadas (republics) according to the Buddhist  text Anguttara Nikkaya and the Jain text Bhagavati  Sutra. 
A Lichchavi confederacy republic  according to Jataka stories had as much as 7707 chiefs or counsels (Rājans as  they called them). A Buddhist document says Lichchavis had 500 Rājans and a  commentary suggests that the chiefs governed in turns. It was the same  Lichchavi republic from where the mother of Jain Thirthankar Mahaveer  originated from.
There is also an example of a  republic in the ‘Vajjian/Vrijjian union’, which had Vaishali (in what is now  Bihar), as its capital around 600 BCE. About this Vajjian union, it was the  great Lord Buddha who said to his chief disciple Ananda thus: “For as long,  Ānanda, as the Vajjians will assemble regularly and assemble frequently surely  growth is to be expected for the Vajjians not decline.” 
At the time of Alexander’s  invasion in 327 BCE into the regions of ancient Afghanistan, the states of Arut  and Balhika were affluent republics. These republics later became allies of the  great Chandragupta Maurya (320 BCE – 298 BCE).  Though Chandragupta Maurya conquered almost  the entire subcontinent, some of his annexed areas continued to function as  republics and returned to being republics after the fall of his empire. Another  case in point is Madra desh in north western India, which survived as a  republic until the 4th century CE. 
How did the republic work…
 While the word for a republic was  sangha or gana as in the “Bharatiya gana-rajya”, some were sovereign in their  origin with no allegiance to any external authority. Though they were not all  of the same type, they had certain features common to all. Each had its council  assembly or parishad where people’s representatives were present from different  age groups. 
The Indian historians R.C.  Majumdar, H.C. Raychaudhuri and Kalikinkar Datta in their seminal work: ‘An Advanced History of India (1946) mentions  that, “Besides the Rājan, there were other functionaries styled uparājan  (Vice-Counsel), Senāpati (general), Bhāndāgārika (treasurer) etc. Tradition  points to the existence of a succession of officials for the administration of  criminal law in the Vrijjan state – the Vinishaya mahāmudra (deciding  magistrates), Vyāvahārika (lawyer-judge), Sūrtadhara (canonist), Ashtakulika  (representative of the eight clans), Senāpati (General), Uparājan  (Vice-counsel), and Rājan (Consul)”
Even today we can see the surviving  village panchayats for whom the government of India has a dedicated ministry of  Panchayati Raj has republican values as its core. In his famous minute of 1830,  Sir Charles Metcalfe, the then acting governor-general of India wrote:  "The village communities are little republics, having nearly everything  they can want within themselves, and almost independent of any foreign  relations..."
In a republic society,  legislation and law making is a collective effort of thinkers and ancient India  was no exception. While a republic evolved as a remedy and revolution to  monarchy in Europe, the concept and practice of a republic was integral to the  Indian system of governance with or without a monarch. In fact, the 26th  January gana-tantra divas (republic day) celebrations should can also be seen  as a continuation of the republican values in the long and chequered history of  India. 
Ram Lingam blogs his insights on India and Indian culture  at www.indiasutra.co.nz
Also read:
Raja Dharma by  Swami Dayanand Sarawati 
Town Planning  Arthashastra